1902.] NATURAL SCIENCES OF PHILADELPHIA. 229 



race, oi- upon the growth-euergies of the race bemg essentially the 

 sum total of the growth-energies of the individuals which compose 

 it ? Evidently the latter, with the reservation that growth- 

 energies are directed to the development of structure immediately 

 in view, and with change of habit become correspondingly modi- 

 fied. If at a certain stage the individual is pelagic and later 

 sessile, the growth-energies are fii'st directed to the formation of the 

 structures necessary for the habits of the pelagic period, later to the 

 formation of those for the sessile condition. If the pelagic habit 

 became lost, the growth-euergies from the first become directed 

 toward the development of structures essential for the sessile state." 

 There is here a continual adaptation to new conditions of life, 

 restricted by the line of direction held by these energies in pre- 

 ceding ontogenies, the restriction more or less directly diminishing 

 according as the preceding ontogeny is far removed, and according 

 to the degree of plasticity of the organization. 



Obviously, in regard to the Annelida and INIollusca, we may 

 conclude either that their cell lineage is so closely correspondent 

 because of their close genetic affinity (there not having been time 

 for a great difference to arise), or because the cell lineage in both 

 is followed by a more or less similar larval stage. In the former" 

 view genetic affinity is postulated, in the latter convergence of 

 cleavage on account of the similarity in the larvae. As to the 

 larva?, these may be alike because oi the genetic affinity of the two 

 groups, or because they lead a corresponding mode of life and are 

 formed at an equally early period. At every point in this discus- 

 sion of embryonic and larval homologies we find the possibility of 

 two divergent opinions. The 2^i'os and cons of the Iwo positions 

 have been more or less fully discussed by the cell liueagists, and it 

 is not for me to enter into questions concerning the homologies of 

 blastomeres. It is sufficient to state hei'e that the cell lineagists 

 have more or less divided opinions on these points, as can be seen 

 by an examination of the papers of E. B. Wilson (different 

 opinions expressed in successive studies), Conkliu, Mead, Tread- 

 well, Lillie, Eisig, Heath, and others. 



One point may be made here. If the biogenetic hypothesis is 



"This is necessarily not an exact statement of the facts. Exactly 

 stated, the growth-energies are immediatehi directed to the establishment 

 of the next following stage. Yet, certainly all the energies are directed 

 toward the establishment of the ultimate structural condition. 



