488 LIFE HISTORY STUDIES OF ANIMALS. 



answering as well as I can a few of the questions which occur to any 

 naturalist who occupies himself with life histories. Even a partial 

 answer, even a mistaken answer, is better than the blank indifference 

 of the collector who records and records, but never thinks about his 

 facts. 



The first question that I will put is this: Why do some animals 

 undergo t^^ansformation while others do not? It has long been noticed' 

 that as a rule fresh-water and terrestrial animals do not go through 

 transformation, while their marine allies do. Let us take half a dozen 

 examples of each : 



Fluviatile or terrestrial, v/itbout trans- Marine, with transformation: 



formation : Crab, 



Crayfish, Polygordius, 



Earthworm, Doris, JEolis, 



Helix, Oyster, 



Cyclas, Most hydrozoa, etc. 

 Hydra, etc. 



We get a glimmer of light upon this characteristic difference when 

 we remark that in fresh- water and terrestrial species the eggs are often 

 larger than in the allied marine forms A large egg favors embryonic 

 as opposed to larval development. An embryo which is formed within 

 a large egg may feed long upon the food laid up for it and continue its 

 development to a late stage before hatching. But if there is little or 

 no yolk in the egg the embryo will turn out early to shift for itself It 

 will be born as a larva, provided with provisional organs suited to its 

 small size and weakness. Large eggs are naturally fewer than small 

 ones. Does the size depend on the number or the number on the size"? 

 To answer in a word, I believe that the size generally depends on the 

 number and that the number is mainly determined by the risks to 

 which the species are exposed. At least so many eggs will in general 

 be produced as can maintain the numbers of the species in spite of 

 losses, and there is some reason to believe that in fresh waters the risks 

 are less than in the shallow seas or at the surface of the ocean.^ In 

 most parts of the world the fresh waters are of small size and much 

 cut up. Every river basin forms a separate territory. Isolation, like 

 every other kind of artificial restriction, discourages coniiDetition and 

 impedes the spread of successful competitors. In the shallow seas or 



1 Darwin, Origin of Species, Chap. XIII; Fritz Mliller, Fiir Darwin, Chap. VII. 



2 Indications are given by the survival in fresh waters of declining groups, e. g., 

 ganoid fishes, which, when dominant, maintained themselves in the sea, and by the 

 not uncommon case of marine animals which enter rivers to spawn. I do not attempt 

 to count among these indications the supposed geological antiquity of fluviatiJe as 

 compared with marine animals. Some marine genera are extremely ancient {Lingula, 

 Nucnia, Trir/onia, Nautilus); a perfectly fair comparison is almost impossible, and 

 great persistence does not necessarily imply freedom from risks. In the mollusca, 

 which afford a good opportunity of testing the effect of habitat upon the number of 

 the eggs, marine species seem to produce more eggs as a rule than fluviatile, and 

 these many more than terrestrial species. 



