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varied. Pasteur has defined fermentation as “life without air." A host of chemical changes in organic matter, fermentation, putrefaction, infection of disease—all these are the work of minute organisms none the less real because invisible and as varied in form and structure as in the differing effects their presence may produce.

Each kind of animal or plant, that is, each set of forms which in the changes of the ages has diverged tangibly from its neighbors, is called a species. There is no absolute definition for the word species. The word kind represents it exactly in common language, and is just as susceptible to exact definition.

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Fig. 2.-Kangaroo rat from the California-Mojave desert (one-half natural size).

The scientific idea of species does not differ materially from the popular notion. A kind of tree or bird or squirrel is a species. Those individuals which agree very closely in structure and function belong to the same species. There is no absolute test, other than the common judgment of men competent to decide. Naturalists recognize certain formal rules as assisting in such a decision. A series of fully intergrading forms, however varied at the extremes, is usually regarded as forming a single species. There are certain recognized effects of climate, of climatic isolation, and of the isolation of domestication. These do not usually make it necessary to regard as distinct species the extreme forms of a series concerned.

In the words of the entomologist Rambur, “A species is a group of beings which in successive generations show the same characters of organization, unchanged so long as the locality and external conditions remain unchanged."

The number of species actually existing is far beyond ordinary conception. The earliest serious attempt to catalogue the species of animals and plants was made by Linnaeus. In the tenth edition of his “Systema Naturae '' in 1758, in the 823 pages

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devoted to animals, he describes and names some four thousand different kinds. Great as this number seemed, Linnaeus ventured to suggest that probably his pages did not include half of those kinds of animals actually existing. To-day our records contain descriptions of more than one hundred and fifty times as many kinds of animals as were known to Linnaeus and all his predecessors and all his associates of a century and a half ago. Each year, since 1864, there has been published in London a volume called the “Zoological Record.” Each of the volumes—larger than the whole “ Systema Natura” -- contains the names of the animals new to science which have been added to the system in the year of which it treats. In the

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record of each year we find about twelve thousand species, about three times as many animals as in the whole “Systema Nature.” Yet the field shows no signs of exhaustion. As the volumes of the “ Zoological Record” stand on the shelves, it is easy to see that the later volumes are the thickest; and those of the new century, with a general revival of interest in systematic zoology

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Fig. 4.-California quail, Lophortyr californicus. (Two-thirds natural size.)

and the study of geographical distribution, are the thickest of all. The depths of the sea, the jungles of the tropies, the crevices of the coral reefs, the tundras of the north, the limbs of trees, the hair of mammals, the feathers of birds, the body tissues of mosquitoes, all places where animal life is found, are being examined with an eagerness not less than that of the early explorers, while the investigators of to-day are armed with every appliance that science can devise. Yet now, as in Linnæus's time, it is certain that not half of the number of species of animal organisms is yet known. The 600,000, more or less,

investigation devise. Yet mber of spe

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Fig. 5.- Diamond rattlesnake, Crotalus adamanteus. (Photograph by W. K. Fisher.)

on our registers to-day are certainly far less than half of the 'millions which actually exist.

In botany we find the same conditions. There are fewer known species of plants than animals by half, and they are more easily preserved and handled, while the work of collection and investigation proceeds on a scale even more extensive, yet it would be a bold statement to say that we know to-day half the species of plants that exist.

All this refers to the forms now living, without reference to the host which composes their long ancestry, extending backward toward the dawn of creation. The species have come down through the geological ages, changing in form and function to meet the varying needs of changing environment. This enumeration takes no account of the still vaster myriads of forms almost endlessly varied which have perished utterly in the pressure of environment, leaving no trace in the line of descent.

Of these extinct forms of animals and plants we know a few, one here and another there: here a bone, there a tooth, here a

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