THE EPIC OF EVOLUTION 



/|87 



The size of an animal may be in itself a structural contribution to 

 success in life. A single horse that weighs a ton and a ton of mice 

 both require in general the same sort and amount of food, but the 



.-..secor^d y77axilla 



maxillary I 

 palp 



-first 

 maxilla. 



— )T7axillar/ palp 



/.mandible. 

 \abr-am.j 

 Second, maxilla 



The cockroach (left) and the female mosquito 

 (right) inherit a homologous set of mouth parts, 

 which have become considerably modified to meet 

 the conditions imposed by different functions. 



mice stand the better 

 chance of getting about 

 and securing food 

 necessary for main- 

 taining a ton of proto- 

 plasm. 



Some other random 

 suggestions of examples 

 of structural adapta- 

 tions are radial sym- 

 metry in sessile ani- 

 mals, the histological 

 structure of leg bones 

 adapted to bear body 

 weight, the handy, 

 prehensile tails of South 

 American monkeys, the 

 sharp claws of certain bloodthirsty carnivores, the sticky protrusible 

 tongue of ant-eaters, the snowshoelike feet of the Mexican jacanas, 

 which get their insect food while skipping lightly over floating lily- 

 pads, the elongated snouts of chestnut weevils that have the problem 

 of spiny burrs to solve, and the shoelike hoofs of heavy ungulates. 



Embryological Adaptations 



Reptiles and birds, that hatch by breaking through an enclosing 

 eggshell after making a preliminary start in life, and mammals, which 

 go through the early stages of their development in safety within 

 the mother's body, have to be fitted successively for two quite dif- 

 ferent sets of conditions. Such embryos, during the period of their 

 imprisonment, employ two notable adaptive devices, the amnion and 

 the allantois, which are discarded upon emergence. The amnion is 

 an enveloping antenatal robe, filled with fluid, within which the deli- 

 cate, rapidly growing embryo floats, protected from mechanical shock 

 and from growth-checking exposure to a dry world. It is an adapta- 

 tion to land life quite unnecessary in the case of fishes and amphibians, 

 whose usually shell-less eggs are deposited in the water during the 

 period of their preliminary development. The allantois is a make- 

 u. w. H. — 32 



