PRESIDENTIAL ADDRESS. 33 



If we are right in interpreting the eggs of Metazoa as representing 

 the unicellular or protozoan stage in their ancestry, then the small size 

 of the egg may be viewed as recapitulatory. 



But the gradual increase in size of the embryo, and its growth up 

 to the adult condition, can only be regarded as representing in a most 

 general way, if at all, the actual or even the relative sizes of the 

 intermediate ancestral stages of the pedigree. 



It is qmte true that animals belonging to the lower groups are, as a 

 general rule, of smaller size than those of higher grade ; and also 

 that the giants are met with among the highest members of each 

 division. Cephalopoda are the highest molluscs, and the largest cepha- 

 lopods greatly exceed in size any other members of the group ; decapods 

 are at once the highest and the largest crustaceans ; and whales, the 

 hugest animals that exist, or, so far as we know, that ever have existed, 

 belong to the highest group of all, the mammalia. It would be easy 

 to quote exceptions, but the general rule obtains admittedly. 



However, although there may be, and probably is, a general 

 parallelism between the increase in size from the egg to the adult, and 

 the historical increase in size during the passage from lower to higher 

 forms ; yet no one could maintain that the sizes of embryos represent 

 at all correctly those of the ancestors ; that, for instance, the earliest 

 birds were animals the size of a chick embryo at a time when avian 

 characters first declare themselves, or that the ancestral series in all 

 cases presented a steady progression in respect of actual magnitude. 



In the lower animals, e.g., in Orbitolites, the actual size of the 

 several ancestral stages is probably correctly recapitulated during the 

 growth of the adult ; and it is very possible that it is so also in such 

 forms as the solitary sponges. In higher animals, except in the early 

 stages of those forms which are practically devoid of food yolk, and 

 which hatch as pelagic larvae, this certainly does not obtain. 



This is clear enough, but is worth pointing out, for if, as most 

 certainly is the case, the embryos of animals are actually smaller than 

 the ancestral forms they represent, it is possible that the smallness of 

 the embryo may have had some influence on its organisation, and be 

 responsible for some of the modifications in the ancestral history ; and 

 more especially for the disappearance of ancestral organs in free 

 swimming larvae. 

 c 



