THE BACKGROUND OF DARWINISM—ADAPTATIONS 197 
in an exceedingly immature state and looking like tiny pink grubs. 
They crawl under their own power, by means of a swimming-like 
motion, through the hairs of the mother’s abdomen, till they reach the 
pouch. This they enter unaided and each tiny larva finds for itself 
a slender tubular teat, which it swallows and holds in place by a 
specially adapted hold-fast mouth. The young remains attached 
fixedly to this teat for some weeks, feeding almost constantly on milk. 
After a long interval the teat is released, the mouth metamorpheses 
into the adult form and the young feeds only at intervals, as do the 
young of other mammals. This complex of adaptive structures and 
instincts is among the most remarkable in the annals of biology. 
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 Jarval 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 
