178 The Study of Animal Life part hi 



robe, and in almost all mammals by part of the placenta 

 which unites mother and unborn offspring. 



Substitution of Organs. — To the embryologist Kleinen- 

 berg we owe a suggestive conception of organic change, 

 which he speaks of as the development of organs by sub- 

 stitution : An organ may supply the stimulus and the 

 necessary condition for another which gradually supersedes 

 and replaces it. In the simplest backboned animals, such 

 as the lancelet, there is a supporting gristly rod along the 

 back ; among fishes the same rod or notochord is largely 

 replaced by a backbone ; in yet higher Vertebrates the 

 adults have almost no notochord, its replacement by the 

 backbone is almost complete. So in the individual life- 

 history, all vertebrate embryos have a notochord to begin 

 with ; in the lancelet and some others this is retained 

 throughout life, in higher forms it is temporary and serves 

 as a scaffolding around which, from a thoroughly distinct 

 embryological origin, the backbone develops. What is the 

 relation between these two structures — notochord and 

 backbone ? According to Kleinenberg, the notochord 

 supplies the necessary stimulus or condition for the 

 development of the backbone which replaces it. 



Rudimentary Organs. — (a) Through some ingrained 

 defect it sometimes happens that an organ does not 

 develop perfectly. The heart, the brain, the eye may be 

 spoilt in the making. Such cases are illustrations of 

 arrested development, [b) A parasitic crustacean, such as 

 the Sacculina which shelters beneath the tail of a crab, 

 begins life with many equipments such as legs, food-canal, 

 eye, and brain, which are afterwards entirely or nearly 

 lost ; the sedentary adult sea-squirt or ascidian has lost the 

 tail, the notochord, the spinal cord which its free-swimming 

 tadpole-like larva possessed. Such cases are illustrations 

 of degeneration. In these instances the retrogression is 

 demonstrable in each lifetime, in other cases we have to 

 compare the animal with its ancestral ideal. Thus there 

 are many cave -animals whose eyes are always blind and 

 abortive. The little kiwi of New Zealand has only apologies 

 for wings. We need have no hesitation in calling these 



