The importance of migration and of the consequent isolation of groups of animals, for the origin of new varieties and ultimately of new species, which was indicated by Moritz Wagner, was fully recognized by Darwin himself. Consequent researches have only accentuated the importance of this factor, and they have shown how the largeness of the area occupied by a given species—which Darwin considered with full reason so important for the appearance of new varieties—can be combined with the isolation of parts of the species, in consequence of local geological changes, or of local barriers. It would be impossible to enter here into the discussion of this wide question, but a few remarks will do to illustrate the combined action of these agencies. It is known that portions of a given species will often take to a new sort of food. The squirrels, for instance, when there is a scarcity of cones in the larch forests, remove to the fir-tree forests, and this change of food has certain well-known physiological effects on the squirrels. If this change of habits does not last—if next year the cones are again plentiful in the dark larch woods—no new variety of squirrels will evidently arise from this cause. But if part of the wide area occupied by the squirrels begins to have its physical characters altered—in consequence of, let us say, a milder climate or desiccation, which both bring about an increase of the pine forests in proportion to the larch woods—and if some other conditions concur to induce the squirrels to dwell on the outskirts of the desiccating region—we shall have then a new variety, i.e. an incipient new species of squirrels, without there having been anything that would deserve the name of extermination among the squirrels. A larger proportion of squirrels of the new, better adapted variety would survive every year, and the intermediate links would die in the course of time, without having been starved out by Malthusian competitors. This is exactly what we see going on during the great physical changes which are accomplished over large areas in Central Asia, owing to the desiccation which is going on there since the glacial period.
To take another example, it has been proved by geologists that the present wild horse (Equus Przewalskii) has slowly been evolved during the later parts of the Tertiary and the Quaternary period, but that during this succession of ages its ancestors were not confined to some given, limited area of the globe. They wandered over both the Old and New World, returning, in all probability, after a time to the pastures which they had, in the course of their migrations, formerly left.59 Consequently, if we do not find now, in Asia, all the intermediate links between the present wild horse and its Asiatic Post-Tertiary ancestors, this does not mean at all that the intermediate links have been exterminated. No such extermination has ever taken place. No exceptional mortality may even have occurred among the ancestral species: the individuals which belonged to intermediate varieties and species have died in the usual course of events—often amidst plentiful food, and their remains were buried all over the globe.
In short, if we carefully consider this matter, and, carefully reread what Darwin himself wrote upon this subject, we see that if the word “extermination” be used at all in connection with transitional varieties, it must be used in its metaphoric sense. As to “competition,” this expression, too, is continually used by Darwin (see, for instance, the paragraph “On Extinction”) as an image, or as a way-of-speaking, rather than with the intention of conveying the idea of a real competition between two portions of the same species for the means of existence. At any rate, the absence of intermediate forms is no argument in favour of it.
In reality, the chief argument in favour of a keen competition for the means of existence continually going on within every animal species is—to use Professor Geddes’ expression—the “arithmetical argument” borrowed from Malthus.
But this argument does not prove it at all. We might as well take a number of villages in Southeast Russia, the inhabitants of which enjoy plenty of food, but have no sanitary accommodation of any kind; and seeing that for the last eighty years the birthrate was sixty in the thousand, while the population is now what it was eighty years ago, we might conclude that there has been a terrible competition between the inhabitants. But the truth is that from year to year the population remained stationary, for the simple reason that one-third of the newborn died before reaching their sixth month of life; one-half died within the next four years, and out of each hundred born, only seventeen or so reached the age
