Natural selection does not create species, it enforces adaptation. If a species or a group of individuals cannot fit itself to its environment, it will be crowded out by others which can do so. It will then either disappear entirely from the earth, or it will be limited to that region or to those conditions to which it is adapted. A partial adjustment tends to become more perfect, for the individuals least fitted are first destroyed in the struggle for existence. Very small variations may sometimes, therefore, lead to great changes. A side issue apparently unimportant may perhaps determine the fate of a species. Any advantage however small may possibly turn the scale of life. "Battle within battles must be continually recurring, with varying success, yet in the long run the forces are so nicely balanced that the face of nature remains for a long time uniform, though assuredly the merest trifle would give the victory to one organic being over another."

Darwin says:

"I have found that the visits of bees are necessary for the fertilization of some kinds of clover; for instance, twenty heads of white clover (Trifolium repens) yielded two thousand two hundred and ninety seeds, but twenty other heads protected from the bees produced not one. Again, one hundred heads of red clover (Trifolium pratense) produced two thousand seven hundred seeds, but the same number of protected heads produced not a single seed. Humble-bees alone visit red clover, as other bees cannot reach the nectar. . . . Hence we may infer as highly probable that, if the whole genus of humble-bees became extinct or very rare in England, the heartsease and red clover would become very rare or wholly disappear. The number of humble-bees in any district depends in a great measure on the number of field mice, which destroy their combs and nests; and Colonel Newman, who has long attended to the habits of humble-bees, believes that more than two-thirds of them are thus destroyed all over England. Now the number of mice is largely dependent, as everyone knows, on the number of cats; and Colonel Newman says: 'Near villages and small towns I have found the nests of humble- bees more numerous than elsewhere, which I attribute to the number of cats that destroy the mice.' Hence it is quite credible that the presence of feline animals in large numbers in a district might determine, through the intervention first of mice and then of bees, the frequency of certain flowers in that district."

Huxley carries this calculation still further by showing that the number of cats depends on the number of unmarried women. On the other hand, clover produces beef, and beef strength. Thus in a degree the prowess of England is related to the number of spinsters in its rural districts! This statement would be true in all seriousness were it not that so many other elements come into the calculation. But whether true or not, it illustrates the way in which causes and effects in biology become intertangled.

There was introduced into California from Australia, on young lemon trees, twenty-five years ago, an insect pest called the cottony cushion scale (Icerya purchasi). This pest increased in numbers with extraordinary rapidity, and in ten years threatened to destroy completely the great orange orchards of California. Artificial remedies were of little avail. Finally, an entomologist was sent to Australia to find out if this scale insect had not some special natural enemy in its native country. It was found that in Australia a certain species of ladybird beetle attacked and fed on the cottony cushion scales and kept them in check (Fig. 39). Some of these ladybirds (Vedalia cardinalis) were brought to California and released in a scale-infested orchard. The ladybirds, having plenty of food, thrived and produced many young. Soon they were in such numbers that many of them could be distributed to other orchards. In two or three years the Vedalias had become so numerous and widely distributed that the cottony cushion scales began to diminish perceptibly, and soon the pest was nearly wiped out. But with the disappearance of the scales came also a disappearance of the ladybirds, and it was then discovered that the Vedalias fed only on cottony cushion scales and could not live where the scales were not. So now, in order to have a stock of Vedalias on hand in California, it is necessary to keep protected some colonies of the cottony cushion scale to serve as food. Of course, with the disappearance of the predaceous ladybirds the scale began to increase again in various parts of the State, but with the sending of Vedalias to these localities the scale was again crushed. How close is the interdependence of these two species!

There is little foundation for the current belief that each species of animal has originated in the area it now occupies, for in many cases our knowledge of paleontology shows the reverse of this to be true. Even more incorrect is the belief that each

species occupies the district or the surroundings best fitted. for its habitation. This is manifested in the fact of the extraordinary fertility and persistence shown by many kinds of animals and plants in taking possession of new lands which have become, through the voluntary or involuntary interference of man, open to their invasion. Facts of this sort are the "enormous increase of rabbits and pigs in Australia and New Zealand, of horses and cattle in South America, and of the sparrows of North America, though in none of these cases are the animals natives of the countries in which they thrive so well" (Wallace). The persistent spreading of European weeds to the exclusion of our native plants is a fact too well known to every farmer in America. The constant moving westward of the white weed and the Canada thistle marks the steady deterioration of our grass fields. The cockroaches in American kitchens represent invading species from Europe. The American cockroaches live in the woods. Perhaps a majority of the worst insect pests of the United States are of European or Asiatic origin. Especially noteworthy are cases of this type in Australia and New Zealand. In New Zealand the weeds of Europe, toughened by centuries of selection, have won an easy victory over the native plants.

Dr. Hooker states that, in New Zealand "the cow grass has taken possession of the roadsides; dock and watercress choke the rivers; the sow thistle is spread all over the country, growing luxuriantly up to 6,000 feet; white clover in the mountain districts displaces the native grasses." The native Maori saying is: "As the white man's rat has driven away the native rat, as the European fly drives away our own, and the clover kills our fern, so will the Maoris disappear before the white man himself." Prof. Sidney Dickinson gives the following notes on the rabbit and other plagues of Australia:

"The average annual cost to Australasia of the rabbit plague is £700,000, or nearly $3,500,000. The work which these enormous figures represent has a marked effect in reducing the number of rabbits in the better districts, although there is little to suppose that their extermination will ever be more than partial. Most of the larger runs show very few at present, and rabbit-proof fencing, which has been set around. thousands of square miles, has done much to check further inroads. Until this invention began to be utilized it was not uncommon to find

as many as a hundred rabbiters employed on a single property whose working average was from three hundred to four hundred rabbits per day. As they received five shillings a hundred from the station owner, and were also able to sell the skins at eight shillings a hundred, their profession was most lucrative. Seventy-five dollars a week was not an uncommon wage, and many an unfortunate squatter looked with envy upon the rabbiters, who were heaping up modest fortunes, while he himself was slowly being eaten out of house and home.

"The fecundity of the rabbit is amazing, and his invasion of remote districts swift and mysterious. Careful estimates show that, under favorable conditions, a pair of Australian rabbits will produce six litters a year, averaging five individuals each. As the offspring themselves begin breeding at the age of six months, it is shown that, at this rate, the original pair might be responsible in five years for a progeny of over twenty millions. That the original score that were brought to the country have propagated after some such ratio, no one can doubt who has seen the enormous hordes that now devastate the land in certain districts. In all but the remoter sections, the rabbits are now fairly under control; one rabbiter with a pack of dogs supervises stations where one hundred were employed ten years ago, and with ordinary vigilance the squatters have little to fear. Millions of the animals have been killed by fencing in the water holes and dams during a dry season, whereby they died of thirst, and lay in enormous piles against the obstructions they had frantically and vainly striven to climb, and poisoned grain and fruit have killed myriads more. A fortune of £25,000 offered by the New South Wales Government still awaits the man who can invent some means of general destruction, and the knowledge of this fact has brought to the notice of the various colonial governments some very original devices.

"Another great pest to the squatters is developing in the foxes, two of which were imported from Cumberland some years ago by a wealthy station owner, who thought that they might breed, and give himself and friends an occasional day with the hounds. His modest desires were soon met in the development of a race of foxes far surpassing the English variety in strength and aggressiveness, which not only devour many sheep, but out of pure depravity worry and kill ten times as many as they can eat. When to these plagues is added the ruin of thousands of acres from the spread of the thistle, which a canny Scot brought from the Highlands to keep alive in his breast the memories of Wallace and Bruce; the well-nigh resistless inroads of furze; and, in New Zealand, the blocking up of rivers by

the English watercress, which in its new home grows a dozen feet in length, and has to be dredged out to keep navigation open, it may be understood the colonials look with jaundiced eye upon suggestions of any further interference with Australian nature.

"Not to be outdone by foreign importations, the country itself has shown in the humble locust a nuisance quite as potent as rabbit, fox, or thistle. This bane of all men who pasture sheep on grass has not been much in evidence until within the last few years, when the great destruction of indigenous birds by the gun and by poisoned grain strewn for rabbits has facilitated its increase. The devastation caused by these insects last year was enormous, and befell a district a thousand miles long and two thousand wide. For days they passed in clouds that darkened the earth with the gloomy hue of an eclipse, while the ground was covered with crawling millions, devouring every green thing and giving to the country the appearance of being carpeted with scales. It has been discovered, however, that before they attain their winged state they can easily be destroyed, and energetic measures will be taken against them throughout all the inhabited districts of Australia whenever they make another appearance.”

The conditions of the struggle for existence are not necessarily felt as an individual stress to the individuals which survive. The life they lead is the one for which they are fitted. The struggle is painful or destructive only to those imperfectly adapted. Men in general are fitted to the struggle endured by their ancestors as they are adapted to the pressure of the air. They do not recognize the pressure itself but only its fluctuations. Hence many writers have supposed that the struggle for existence belongs to animals and plants and that man is or should be exempt from it. Competition has been identified with injustice, fraud, or trickery, and it has been supposed that it could be abolished by acts of benevolent legislation. But competition is inseparable from life. The struggle for existence may be hidden in social conventions or its effects more evenly distributed through processes of mutual aid, but its necessity is always present. Competition is the source of all progress.

The first suggestion of the doctrine of natural selection came to Darwin through the law of population as stated by Thomas Malthus. The law of Malthus is in substance as follows: Man tends to increase by geometrical ratio-that is, by multiplication. The increase of food supply is by arithmetical