414 RECAPITULATION. Cuap. XIV. 



(lep^ree sterile, for their constitutions can hardly fail to bo dis- 

 turbed from beinf^ compounded of two distinct organizations ; 

 but -whether this is the true cause of their sterility I will not 

 pretend to decide. The above parallelism is supported by 

 another parallel, but directly opposite, class of facts, namely, 

 that the vig'or and fertility of all organic beings are increased 

 by slight changes in their conditions of life, and that the 

 oifspring of slightly-modified forms or varieties when crossed 

 acquire increased vigor and fertility. So that, on the one 

 hand, a considerable change in tlie conditions of life and crosses 

 between greatly-modified forms, lessen fertility; and, on the 

 other hand, lesser changes in the conditions of life and crosses 

 between less-modified forms, increase fertility. 



Turning to geographical distribution, the difficulties en- 

 countered on the theory of descent with modification are seri- 

 ous enough. All the individuals of the same species, and all 

 the species of the same genus, or even higher group, must have 

 descended from common parents; and therefore, in however 

 distant and isolated parts of the world they may now be found, 

 they must in the course of successive generations have travelled 

 from some one point to all the others. We are often wholly 

 unable even to conjecture how this could have been effected. 

 Yet, as we have reason to believe that some species have 

 retained the same specific form for very long periods of time, 

 immensely long as measured by years, too much stress ought 

 not to be laid on the occasional wide diffusion of the same spe- 

 cies ; for during very long periods there will always be a good 

 chance for wide migration by many means. A broken or in- 

 terrupted range may often be accounted for by the extinction 

 of the species in the intermediate regions. It cannot be de- 

 nied that Ave arc as yet very ignorant of the full extent of the 

 various climatal and geographical changes which have affected 

 the earth during modern periods; and such changes may obvi- 

 ously have facilitated migration. As an exam])le, I have at- 

 tempted to show how potent has been the influence of the 

 Glacial period on the distribution of the same and of allied 

 species throughout the world. We are as yet profoundly 

 ignorant of the many occasional means of transport. With 

 respect to distinct species of the same genus inhabiting distant 

 and isolated regions, as the process of modification has neces- 

 sarily been slow, all the means of migration will have been 

 possible during a very long period ; and consequently the ■ 



