116 SOLLAS On the Origin of Freshtvater Faunas. 



clearly exhibited ; for the energy which would be expended by a free larva in 

 activities other than those involved in producing structural change is here solely 

 devoted to that end, and hence the embryonic stages are passed over by secluded 

 forms with comparative rapidity. Thus, while it takes several days to produce 

 no higher form than an Echinus from its larva, the chick repeats its enor- 

 mously longer ancestral history in so short a time as three weeks! Thus we 

 find an explanation for another generalization, viz., that the higher the organism 

 the greater the tendency to pass rapidly through its embryological development, 

 involving the abbreviation of one or more of its larval states, or it may even be 

 their total suppression. That it should take nine months for the development of 

 a man and only three weeks for that of a chick may seem strong adverse com- 

 mentary upon this statement ; but we have here to take into account not only the 

 much higher state of organization in which the young child is born, but also its 

 much greater relative size and the lower temperature at which the development is 

 conducted. But it is in the Mammalia that the most perfect method is met with 

 for insuring to the embryo immunity from all work but that of development. 

 For here there is no useless expenditure of energy in maintaining temperature or 

 carrying on respiration. Nourishment is obtained without the intervention of the 

 stomach by direct absorption in the veins, and thus the ancestral history vastly 

 longer in the Mammalia, especially in the higher orders of Mammalia, than in the 

 other groups, is repeated in a conveniently short period.* 



With this kind of intra-uterine development is probably connected the develop- 

 ment of mammse, since the young embryo when born possesses an unused 

 stomach, and must, consequently, if no other reason existed, be dependent for a 

 while upon the mother for appropriate support. In some of the elasmobranch 

 fishes, where intra-uterine development likewise occurs, nourishment is obtained at 

 first from yelk, and subsequently from the vessels of the yelk-sac, and hence the 

 stomachal parts being put to some use in the course of development, there is no 

 need for a digestive education, and the young dog-fish may obtain food for itself 

 as soon as born. 



In the Sauropsida the embryos are nourished by the yelk of the yelk-sac, 

 which, however, is partly employed in supplying blood to the blood-vessels, but it 

 remains quite possible that some of it is directly absorbed by the walls of the 

 alimentary canal. Thus it happens that many birds, as soon as hatched, fall to 

 picking up grain and feed upon it ; but certain exceptions even here seem to 

 show the advantage of a gastric education, for some graminivorous birds, such as 

 pigeons, do not take at once to a grain diet, but are fed by the mother bird with a 

 kind of milk, which is secreted by her crop. So, too, the flamingo pours down 



* The earlier phases are passed through more rapidly than the later, probably owing to tlieir having 

 been far more frequently repeated. 



