BIOLOGICAL INVESTIGATIONS. 145 



to the lecturer, for example, that the real relations of the blas- 

 tula to the gastrula stage should be solved first by observations 

 made strictly within the limits of some one type, and these be 

 checked off and corrected by reference to the accepted or deter- 

 minable position of each animal in its own genetic stock. Also 

 that no conclusion could be accurately drawn from these data 

 until the different branches of the animal kingdom had been 

 reviewed in the same way, especially those in which, like the 

 Cephalopoda and Vertebrata, the earliest stages are highly 

 tachygenic and also complicated by the addition of a food yolk 

 or other introduced characters. 



So far as the position taken in this paper would be likely to 

 affect such questions, a very good argument can be made for 

 the opinion that the gastrulas formed by the bending in of one 

 pole of the blastula are primitive, and other processes are 

 more or less tachygenic forms of this one. Whether this be 

 accepted or rejected does not, however, invalidate the assump- 

 tion made here, which is that the search for the solution of all 

 questions relating to the study of phylogeny can hope for suc- 

 cess only upon the basis of serial comparison, since this is the 

 only method strictly in accord with the principles of evolution 

 and with the methods that have been so efficient in the investi- 

 gations of the laws of development of the ontogeny. 



All of the above remarks tend one way when considered 

 from a general point of view. First, in the study of organ- 

 isms we are dealing with plastic substances showing definite 

 gradations which can be arranged in their natural radio-serial 

 divergent lines of classification, either as they appear on the 

 present surface, or upon a succession of levels in past faunas. 

 Second, there are notable and essential differences in the 

 behavior and structure of the beginnings of any single genetic 

 series as compared with its middle terms, and marked differ- 

 ences also between these same middle terms and the final or 

 aberrant terms, or branches of the same series. Third, it is 

 also asserted that the difference between the extremes and the 

 means of any one genetic series is proportional and usually so 

 great that the grade and position of an organism must be 

 taken into account in every phylogenetic investigation. The 



