THE BIOGENETIC LAW 165 



of development. For the higher the ratio of elimination the more 

 eggs the female must produce, and the more eggs that have to be 

 produced the smaller will be the quantity of nutritive material for the 

 building up of the young embryo which each egg can be furnished 

 with. I know of no records in regard to the eggs of that Brazilian 

 shrimp in which embryonic development ends with the nauplius 

 stage, but we shall certainly not be wrong in predicting that the eggs 

 in this case will be very small and very numerous, in contrast to 

 those of the freshwater crayfish, which are large and, as compared 

 with others known to us, not very numerous. 



It is a point of undeniable theoretical significance which the life- 

 histories of these Crustaceans disclose, that embryogenesis is not con- 

 densed according to hidden internal laws when the structure increases 

 in complexity, but that the condensation of the ontogenetic stages 

 depends upon adaptation, and may be quite different in nearly related 

 species. It shows us anew that all biological occurrences are dominated 

 by the process of selection. 



I have already mentioned that exceptions to the usual mode of 

 development occur even among the lower Crustaceans, and I was 

 thinking at the time of the Daphnids, which leave the egg as fully 

 formed little animals, already equipped with all their segments and 

 limbs. The nauplius stage is passed through in the egg, and it is 

 an interesting indication that the ancestors of the modern species 

 were in the way of moulting, that this embryo nauplius moults within 

 the egg by forming a fine cuticle which is shed after a time. 

 If it be asked why there should be direct development in the case of 

 these small and not very complex water-fleas, while related species, 

 the Branchiopods, which are much richer in segments and in limbs, 

 should emerge from the egg in the form of a nauplius, and then pass 

 thi'ough a longer larval period, we may answer that the reason 

 probably lies in the fact that, in the former case, very few eggs are 

 produced, sometimes only one, often two, seldom more than a dozen, 

 that these eggs can thus be relatively well equipped with yolk, 

 and that the formation of the little body which bears only from 

 seven to nine pairs of limbs can be easily completed within this egg. 

 Other things being equal, the direct development would always be an 

 advantage, because reproduction can begin sooner in the young genera- 

 tion and the number of individuals will thus increase more rapidly. 

 And this is of particular importance in the case of the water-fleas. 



But if it be asked, further, why so few eggs are produced in this 

 case, and whether these animals have no enemies, we must answer that, 

 on the contrary, they are preyed upon and eaten in thousands by 



