SECT, i] PHYSICO-CHEMICAL SYSTEM 317 



that of the sea. But even these two causes together cannot fully 

 account for the phenomenon, for there are many cases of individual 

 species which they will not cover; thus the Cephalopods, which hatch 

 out as minute but very active copies of their parents, i.e. which pass 

 their larval stage within the egg, and which should therefore be 

 immune from the disadvantage described by Sollas, never penetrated 

 into fresh water. 



A third reason must be added to those of Sollas and of von Martens. 

 As will be shown in Sections 12 and 13 the marine invertebrate embryo 

 depends largely on the salts of the sea water for its supply of ash, and 

 therefore could not be expected to develop in a medium very poor 

 in inorganic matter. Colonisation of the fresh water could not occur, 

 then, until animals had begun to provide in each egg sufficient ash 

 to make one finished embryo. There seem to be few data concerning 

 the capacity of marine invertebrate eggs to develop in fresh water, 

 although the adult animals have been found often enough to ac- 

 custom themselves to a fresh-water environment (see the instances 

 given in Semper) . Many studies of the effect of hypotonic solutions 

 on marine embryos can, however, be called to mind, and in all the 

 cases the results are teratogenic. 



The fate of the Cephalopods, it is interesting to note, is explained 

 by this third factor, for Ranzi has demonstrated the intake of the 

 salts in the sea water by the octopus egg. 



As for the general statement that animals can afford their young 

 a better chance of survival by providing them with larger amounts 

 of yolk and therefore a longer incubation-period, there is a striking 

 parallel here with the seeds of leguminous plants which are packed 

 with nourishment. In the Origin of Species (6th ed. p. 56), Darwin 

 wrote, "From the strong growth of young plants produced from such 

 seeds as peas and beans when sown in the midst of long grass, it may 

 be suspected that the chief use of the nutriment in the seed is to 

 favour the growth of the seedlings, whilst struggling with other 

 plants growing vigorously all round". 



It is interesting that the birds show an adaptation exactly similar to 

 the poecilogony of the invertebrates and fishes. Tree-nesting birds are 

 usually nidicolous, but the defenceless state of the newly-hatched squab 

 has brought it about that ground-nesting birds are usually nidifugous. 



As Table 30 shows, the composition of the eggs of all animals other 

 than those of the frog, the silkworm, and certain fishes, is still, to 



