COMPARATIVE ANATOMY AND GENERAL BIOLOGY 331 



development must be relied on for light in certain places, whereas 

 adult structure will be the safer guide in others. Fifteen years ago 

 Gegenbaur said, speaking as a comparative anatomist: "Ohne die 

 Kenntniss des letzeren [i. e., adult structure] wie die Anatomic ihren 

 darstellt, wiirde die Ontogenie sich auf gleichem Wege befinden, 

 wie der Wanderer der sein Ziel nicht kennt." 



More recently Driesch, and still later E. B. Wilson, O. Hertwig, 

 and others, coming at the matter from the embryological side, have 

 insisted upon the importance of what Driesch has aptly styled the 

 prospective value of parts of even the very early embryo, in ques- 

 tions of homogeny. With the common truth underlying these two 

 formulations firmly grasped; and with the principles of develop- 

 mental mechanics thoroughly applied to embryology and compara- 

 tive anatomy alike, through both experiment and the study of 

 nature's own experimenting, one might confidently predict good 

 progress for the future in deepening insight into the past evolution- 

 ary career of the animal world. 



My remaining points, more than the first, are attempts to state 

 certain general biological principles that may serve as more or less 

 reliable guides in handling isolated cases. I earnestly hope, however, 

 to avoid the misfortune of being understood to suppose that I have 

 discovered any laws that run on by some mysterious power all their 

 own, bending the incidents of animal structure to their own ends, 

 whether or no. All, of course, I am attempting is to formulate the 

 concordant results of numerous widely separated observations. If 

 there is anybody in the world who has reason to be skeptical of the 

 invariableness of laws in the realm of living things, it is the zoologist. 



(6) My second point, then, has to do with what was called by 

 Cope the "Law of the Unspecialized; " and also with the law or 

 principle of change of function, first brought into prominence by 

 Dohrn. ' It may be stated thus: The probability that an organ or part 

 in one group of animals has arisen from another in another group by 

 change of function, is inversely proportional to the degree of specializ- 

 ation of the supposed ancestral organ or part. 



I cannot but believe that had this generalization been clearly 

 before the minds of several zoologists who have in recent years 

 advanced theories as to the origin of various groups of animals, 

 they would never have become sponsors for views with which they 

 now stand credited. One may instance Dohrn's attempt to derive 

 the vertebrate copulatory organs from annelid gills; and any num- 

 ber of Gaskell's laborious efforts to show that the various organs of 

 the vertebrate have been derived from organs highly specialized 

 for wholly different functions, in the supposed arthropod ancestor. 

 The role of comparative anatomy in applying this principle is obvious. 

 Grade of specialization can. of course, only be tested by consider- 



