228 PROCEEDINGS OF THE ACADEMY OF [April, 



it may become n circHlns vitiosus ; but at the same time the main 

 principle of comparative auatomy is also a postulate, namely, that 

 essential agreement of adult structure denotes phyletic relationship. 

 When the conclusions from the embryological basis do not agree 

 with those from the anatomical, how is the decision to be made ? 



In the first place, it is evident that the adult stages of organisms 

 do represent phyletic stages, while in many cases ontogentic stages 

 certainly do not. There can also be no doubt that the characters 

 of the adult become " segregated " (Laukester) or " accelerated" 

 (Cope) gradually further back into the ontogeny, as the racial pro- 

 gress continues, and so in the course of time Avould modify more and 

 more the ontogeny as a record of the phylogeny. AJso the sequence 

 in the formation of the organs of the individual need not be the 

 same as their sequence in the racial development ; a point already 

 made by von Baer, and to-day particularly insisted upon by Keibel 

 and others. 



In the second place, the individual nnist be classified at its ma- 

 ture stage in order to show its full degree of racial development. 

 This principle, therefore, demands that the organization should be 

 primarily considered at that stage. Judged from this point of view, 

 any classification based entirely upon the mode of cleavage of the 

 ovum or upon the occurrence of a certain kind of larva is incor- 

 rect, because it would represent the organism before it had reached 

 its full specialization. Here come such classifications as that given 

 by Friedrich Brauer for the Diptera (according to the position of 

 the split in the shell of the pupa), and that which has led to the 

 formation of the group of the Trochozoa. 



Thirdly, we have to consider on what the correspondence of the 

 ontogeny to the phylogeny rests. The facts of comparative anat- 

 omy show that the groups of the Annelida and MoUusca are phy- 

 letically connected. The careful studies of the cell lineage of types 

 of these two groups have shown a surprising and wholly unexpected 

 degree of correspondence in their mode of cleavage, up to the 

 time of the appearance of the lai^val organs ; Avhile from another 

 standpoint the larval types of both show close structural simi- 

 larity. This is sufficient as a case for examination, though a 

 number of others would suffice equally well. Now, does this cor- 

 respondence of the ontogeny to the phylogeny depend upon 

 the individual inheriting the successive preceding stages of the 



