546 POPULAR SCIENCE MONTHLY 



ity, or certain substances in solution, supplied at the proper moments. 

 The obvious suggestion is, that species are more readily modified than 

 is commonly admitted ; and that in particular they are likely to be 

 so modified on the borders of the territory they occupy, where they 

 continually impinge on unaccustomed environments. 



To show that this is possible is a most important step; but we still 

 have to enquire how far has it actually occurred? In the case of our 

 flax, we have an excellent example of the production of a new form on 

 the periphery of the old, permitting expansion through modification; 

 but only one such derivative seems to have been produced. In other 

 instances, as the experimentalists have shown, the apparent instances 

 are illusor}^ the supposed geographical segregates being merely exam- 

 ples of a single type variously modified by the direct action of the 

 environment. The most striking evidence of this sort has been fur- 

 nished by Beebe, who has produced in certain birds, by means of humid- 

 ity, more difference that has been accepted as sufficient for the dis- 

 tinction of subspecies. Leaving out all such phenomena, we still have 

 a great series of closely allied species, with undoubtedly inherited char- 

 acters, presenting the same kinds of differences as have been observed 

 to arise by mutation, sometimes apparently as the direct result of par- 

 ticular stimuli. What do these phenomena mean in the practical work- 

 ing out of evolutionary processes? 



If we know in a general way the age of particular types and the 

 extent of their migrations, we can begin to form an idea of their prac- 

 tical mutability. The vertebrate paleontologist finds evidence of re- 

 markable changes within the Tertiary period, but even he has to admit 

 that the course of evolution is not so rapid as it might seem ; that new 

 forms suddenly appearing must surely have migrated from other 

 regions, where they doubtless underwent a slow process of development. 

 Central Asia, we must now think, must have been the home of various 

 groups, and will one day yield fossils of surpassing interest. Africa, 

 once seeming so barren paleontologically, has of late begun to yield 

 her treasures. 



The student of fossil invertebrates finds the process of change to 

 have been, in the majority of cases, extraordinarily slow ; and the paleo- 

 botanist finds it slower still. It is not that plenty of specific forms 

 were not produced, but the generic and higher types were so little sus- 

 ceptible to change. A wrong impression has been produced, even 

 among the vertebrates, by the presence in different strata of remarkable 

 extinct groups. No doubt two or three species of elephants walking 

 about in our mountain-parks would give a strikingly different appear- 

 ance to the Colorado landscape; but the time since this actually occurred 

 is, geologically speaking, very inconsiderable, and does not represent 

 any great step in the process of evolution. The more I study the 



