16 The Mechanism of Evolution" in Leptinotaesa 



These products of mutation, saltation, and germinal disintegration are largely 

 of laboratory origin, and at present there is entire ignorance in the main as to 

 how these products may behave in nature ; and it is not known whether they 

 could or would become the progenitors of specific groups. This is not an 

 impossible subject for investigation, and it should receive experimental investi- 

 gation by the introduction of newly arising groups into localized natural 

 habitats, and their behavior studied through a series of years. In this manner 

 it will be possible to evaluate the different agencies concerned with respect to 

 their efficiency in the production of diversity in nature. 



ISOLATION AND SEGREGATION. 



Two other hypotheses call for mention at this place, namely, the supposed 

 action of isolation and segregation so ardently advocated by Warming, Wagner, 

 Gulick, Jordan, and others. This hypothesis of species formation by isolation 

 is little more than the plausibilities of geographical distribution and faunistic 

 study and the juxtaposition of possibly unrelated facts observed in nature, with 

 the presentation of them interwoven into an hypothesis of natural causation, 

 in which the fact of their being isolated from one another is conceived of as the 

 efficient motive force. Within this group of writers one finds various causes 

 asserted to be the agency which produces the modification supposed to result 

 from isolation and environmental actions, the various types of isolation elabo- 

 rated by Gulick, and many others, wherein conceivable but unproven agencies 

 are called into action to explain the conditions found in nature. Habitudinal 

 geographic factors may well be effective agencies in the production of hetero- 

 geneity in nature, but these need to be investigated by experimental methods, 

 and I have found in some of the species that I have used that there were degrees 

 of geographical heterogeneity associated to a greater or less extent with different 

 habitat complexes surpassing in refinement that recorded by any worker. All of 

 these differences, however, are entirely the product of distinct gametic factors, 

 which can be determined and experimented with through modern methods. I 

 have likewise attempted preliminary experimentation in nature in the effort to 

 create isolated colonies of species of like gametic constitution, and then to test 

 repeatedly the nature of these species in the effort to determine whether isola- 

 tion is productive of change, and, if so, to what cause the changes are due. Thus 

 far the results obtained show that isolation alone is not productive of germinal 

 changes, provided that homogeneous materials are used for the experiment and 

 as long as the new conditions do not operate as effective factors in the production 

 of gametic changes by the direct action of the environmental conditions. Isola- 

 tion unaccompanied by such environmental action never shows, even with the 

 closest inbreeding, any change in characters, and there is no evidence that with 

 gametically uniform stock there is any possibility of such processes resulting in 

 change. When stocks are not uniform or pure, then there have resulted isolated 

 races in these experiments, and this experimental production of them may help 

 in interpreting the conditions in nature, and aid in the solution of the origin of 

 many geographical races. 



ORTHOGENESIS. 



Lastly, there is the hypothesis of evolution which is characterized " ortho- 

 genesis." It is notorious that, since the time of Aristotle, through the Middle 

 Ages, in the recent writings of Nageli, Eimer, Hyatt, Jackson, Whitman, Osborn, 



