PRESIDENTIAL ADDRESS. 15 



There is no doubt that abundance of food yolk is a direct and very 

 frequent cause of the omission of ancestral stages from individual 

 development ; but it must not be viewed as a sole cause. It is quite 

 impossible that any animal, except perhaps in the lowest zoological 

 groups, should repeat all the ancestral stages in the history of the race ; 

 the limits of time available for individual development will not permit 

 this. There is a tendency in all animals towards condensation of the 

 ancestral history, towards striking a direct path from the egg to the 

 adult. 



This tendency is best marked in the higher, the more complicated 

 members of a group, i.e., in those which have a longer and more 

 tortuous pedigree ; and though greatly strengthened by the presence 

 of food yolk in the egg, is apparently not due to this in the first 

 instance. 



Thus the simpler forms of Orbitolites, as 0. temtissima, repeat in 

 their development all the stages leading from a spiral to a cyclical 

 shell ; but in the more complicated species, as Dr. Carpenter has 

 pointed out, there is a tendency towards precocious development of the 

 adult characters, the earlier stages being hurried over in a modified 

 form ; while in the most complex examples, as in 0. complanata, the 

 earlier spiral stages may be entirely omitted, the shell acquiring almost 

 from its earliest commencement the cyclical mode of growth. There is 

 no question here of relative abundance of food yolk, but merely of early 

 or precocious appearance of adult characters. 



The question of the relations and influence of food yolk, involving as 

 it does the larger or smaller size of the egg, is, however, merely a 

 special side of the much wider question of the nutrition of the embryo, 

 one of the most potent of the disturbing elements affecting develop- 

 ment. 



Speaking generally, we may say that large eggs are more often met 

 with in the higher than the lower groups of animals. Birds and Reptiles 

 are cases in point and, if Mammals do not now produce large eggs, it is 

 because a more direct and more efficient mode of nourishing the young 

 by the placenta has been acquired by the higher forms, and has 

 replaced the food-yolk that was formerly present, and is now retained in 

 quantity by Monotremes alone. Molluscs afford another good example, 

 the eggs of Cephalopoda being of larger size than those of the less 

 highly organised groups. 



