EVOLUTIONAL ZOOLOGY 2O7 



records of the past that at best we can but express the prob- 

 abilities of relationships, and, in picturing to our minds the 

 course of descent, we must call to our aid hypotheses, incap- 

 able of verification, at every step. In not a single instance 

 can we state with assurance what the exact ancestral rela- 

 tionships are that exist between the great groups of inverte- 

 brate animals, to nearly one of which, furthermore, one or 

 another historical evolutionist has accorded the honor of hav- 

 ing given birth to the vertebrates. The results of these his- 

 torical speculations are represented in our present systems of 

 classification which are attempts to express the probabilities 

 of genealogical relationship and lines of descent. 



Little light has been thrown by speculations of this kind 

 upon the causes of evolutionary change or upon the manner 

 in which the individual steps have been taken in the produc- 

 tion of new species. The active workers in biological fields 

 today, after the long period of evolutionary speculation with 

 its unsatisfying discussions and uncertain results, have turned 

 in revolt from the historical method in which they have largely 

 lost their interest, and are now investigating, chiefly by experi- 

 mental means, the nature of those variations in living animals 

 and plants which furnish the materials of evolutionary change 

 and afford contemporary illustrations of the exact manner in 

 which the individual steps of evolution have been taken in 

 the past. This change of attitude has been largely due to 

 the more definite recognition of the fact that the essential 

 aspects of evolution are variation and heredity and that the 

 solution of the interrelationship of species is contained in an 

 understanding of the laws of hereditary transmission. By the 

 application of the experimental method to the study of hered- 

 ity, certain remarkable discoveries have been made, and from 

 these there has sprung a new science the physiology of he- 

 redity, or Genetics, as it is now more generally designated 

 which, from its far-reaching importance occupies today a cen- 

 tral position in biological inquiry. 



