834 REPORT—1890. 
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; ze., in those which havea 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 O. tenwissima, repeat in their develop- 
ment 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 O. 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 development. 
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. 
The large size of the eggs of Elasmobranchs, and perhaps that of Cephalopods 
also, may possibly be associated with the carnivorous habits of the animals; for it 
is of importance that forms which prey on other animals should hatch of con- 
siderable size and strength. 
The influence of habitat must also be considered. It has long been noticed as 
a general rule that marine animals lay small eggs, while their fresh-water allies 
have eggs of much larger size. The eggs of the salmon or trout are much larger 
than those of the cod or herring ; and the crayfish, though only a quarter the length 
of a lobster, lay eggs of actually larger size. 
This larger size of the eggs of fresh-water forms appears to be dependent on the 
nature of the environment to which they are exposed. Considering the geological 
instability of the land as compared with the ocean, there can be no doubt that the 
fresh-water fauna is, speaking generally, derived from the marine fauna; and the 
great problem with regard to fresh-water life is to explain why it is that so many 
groups of animals which flourish abundantly in the sea should have failed to 
establish themselves in fresh water. Sponges and Ceelenterates abound in the sea, but 
their fresh-water representatives are extremely few in number; Echinoderms are 
exclusively marine: there are no fresh-water Cephalopods, and no Ascidians; and 
of the smaller groups of Worms, Molluscs, and Crustaceans, there are many that do 
not occur in fresh water. 
Direct experiment has shown that in many cases this distribution is not due to 
inability of the adult animals to live in fresh water; and the real explanation 
appears to be that the early larval stages are unable to establish themselves under 
such conditions. This interesting suggestion, which has been worked out in detail 
by Professor Sollas,! undoubtedly affords an important clue. To establish itself 
1W. J. Sollas, ‘On the Origin of Freshwater Faunas,’ Scientific Transactions of 
the Royal Dublin Socicty, vol. iii. ser. 11, 1886. 
