356 EVOLUTION, GENETICS, AND EUGENICS 



The fetal membranes of higher mammals constitute one of the 

 most efficient adaptive complexes known. Surrounding the embryo is 

 a fluid-filed sack (amnion) which furnishes an aquatic environment for 

 the soft and delicate body, preventing harmful contacts and allowing 

 ample free space for expansion. The placenta is a co-operative struc- 

 ture, developed out of both fetal and maternal materials, that furnishes 

 an excellent medium for nutritive and other metabolic exchanges be- 

 tween mother and fetus. Although there is no direct vascular connection 

 between them, the mother gives of her nutritive materials to the fetus 

 and takes up from the fetus and eliminates its wastes. As an adapta- 

 tion for carrying out an intricate set of physiological exchanges between 

 two otherwise entirely separate individuals the placenta is unexcelled. 



Nest-making instincts in birds represent, on the behavior side, 

 adaptations of extraordinary perfection. Some nests are built with 

 the greatest care and precision, others represent a relatively crude and 

 slovenly performance. Some nests are made of twigs, fibres, and mud, 

 others of mud alone, still others are hollowed out in clay or sand banks, 

 and some are made in holes in the ground. In any case, the type of 

 nest is highly specific and due to a hereditary instinct; for birds 

 receive no instruction in nest-making. 



Before bringing to a close this brief list of particularly noteworthy 

 adaptations let us recall to mind the series of special adaptations listed 

 as examples of the laws of adaptation, such as aquatic, arboreal, cur- 

 sorial, flying, burrowing, ant-eating, and, especially, adaptations of 

 deep-sea animals. 



PARASITISM AND DEGENERATION 



A vast number of animals and plants have given up the active 

 search for food and have taken up the relatively easy habits of para- 

 sitism. In adaptation to this life certain structures have developed 

 and many of the characters found in independent, free-roving crea- 

 tures have disappeared or become reduced to mere vestiges. Thus 

 the more completely dependent or parasitic an animal becomes, the 

 more completely does it lose its organs of locomotion and its sense 

 organs such as eyes, auditory organs, tentacles, etc. Some animals 

 are free-living when young or in the larval condition and only settle 

 down to a parasitic life when near the end of the life-cycle; other 

 animals are parasitic only when young or larval and become inde- 

 pendent in the adult condition; still others are parasitic throughout 

 the entire life-cycle and pass from host to host without any interval 

 of independent life. Some of these complete parasites pass one phase 

 of the life-cycle on one species of host and the remainder on another 



