of the Costce in the PerisphinctinxE. 453 



observe phyletic trends when he considers a species in 

 its geographic distribution or studies the variations in 

 different characters in successive generations through- 

 out a period of years. In the case of geographic distri- 

 bution, if the zoologist starts from a given center of 

 distribution and is able to follow out in unbroken lines 

 the variations which arise in passing from the point of 

 origin to provinces successively further removed, then he 

 may, and theoretically should, observe orthogenetic 

 changes. The practical difficulty frequently presents 

 itself of determining to his own and others' satisfaction 

 what was the center of distribution and which was the 

 fundamental primitive species whence the numerous 

 adaptively radiating variations were derived. In the 

 study of determinate variations in a single species the 

 zoologist probably has his best opportunity to see the 

 facts of orthogenesis. As an instance of this method of 

 approach may be cited the observations made by Kellogg 

 and Bell on the flower beetle, Diabrotica soror, collected 

 from the campus of Leland Stanford University. Series 

 of a thousand individuals each were studied and their 

 variations over a period of seven years were recorded 

 and it was found that in that time there had been a very 

 definite change in the prevalence of one type of color 

 pattern on the wings over another. In such a case as 

 this the zoologist, while seeing nothing of ontogeny, yet 

 has the advantage of the time element which is essential 

 in the study of phylogeny, but the time at his disposal is 

 so short that he can observe only comparatively insignif- 

 icant variations in characters sufficiently mutable to 

 show a recognizable change during a period which must, 

 after all, be considered as only an instant when compared 

 with the hundreds of millions of years during which 

 evolution has been producing the complex types which 

 the zoologist observes. Yet, however small such varia- 

 tions are, if they show a definite direction the fact of 

 orthogenesis is clearly demonstrated. Obviously the 

 facts of phylogeny as a whole or even in any appreciable 

 amount are never visible to the zoologist who is mortal. 



The embryologist likewise is cut off from the broad 

 facts of phylogeny because he must of necessity deal with 

 living forms. Furthermore, although he has before him 

 the entire ontogeny of the individuals of the present, he, 

 as a rule, elects to look at only the embryonic stages, 



