INTRODUCTION. XV 



We speak of liiglily-clevelopeil organs, and those which are aborted, atrophied, or 

 rudimentary. Highly-developed organs are also highly specialized or differentiated ; 

 sucli organs are complicated, owing to the distinct uses or divisions of labor accorded 

 to each part. Thus, the eyes of worms are very simple and lowly developed com- 

 pared with tiie hum.an eye; a fish's fin is, in part at least, morphologically the same 

 as the fore-leg of a eat, or the arm of a monkey or of man, but in the human arm 

 different uses are assigned to different portions of the limb; it is highly developed, 

 specialized, or differentiated, to use terms nearly synonymous. 



On the other hand, so exquisitely wrought an organ as a fish's ej-e may by disuse 

 become nearly atrophied, as in the blind fish of Mammoth Cave, which lives in perpet- 

 ual darkness. And not only the eye, but the optic lobes and optic nerves may, as in the 

 case of a small crustacean {Cecidotma sti/gia), also living in Mammoth Cave, be 

 entirely aborted. Among reptiles are some extraordinary cases of modification by 

 degeneration and atrophy. The lizard-like creature, Se2)S, has remarkably small limbs, 

 and in Jiipes there is only a pair of stumps, representing the hinder limbs. As Lan- 

 kester claims, these two forms represent two stages of degeneration or atrojjhy of the 

 limbs ; " they have, in fact, been derived from the five-toed, four-legged ordinary lizard 

 form, and have nearly or almost lost the legs once possessed by their ancestors." The 

 entire order of snakes is an example how the loss, by atroj^hy, of tlie limbs may become 

 common to an entire group of animals numbering thousands of species ; the possession 

 by the boas of a rudimentary pelvis, and minute but nearly atrophied hind legs, tends 

 to prove that all tlie snakes are descendants from some ancestral form whose limbs 

 became lost through disuse. 



Among the Diptera, which have but a single pair of wings, there is an universal 

 atroiihy of the second or hinder pair of wings ; moreover, there are numerous wing- 

 less, degraded forms, and when we take into account the fact that almost all dipterous 

 larvfE are nearly headless and evidently degenerated forms, we are inclined to tliink 

 that the entire grouji of true flies, numbering at least twenty thousand species, are 

 the result of a retrograde development, affecting in every species the hinder pair of 

 wings, and in numerous other forms the niouth-jiarts and other portions of the body, 

 botli in the larval and adult states. The grouji of barnacles (Cirripedia) is another 

 examjile where atrophy and degeneration pervade each member of an order, and the 

 cases are highly interesting and suggestive. 



An entire sub-kingdom of animals may be degenerated in some respects. Such is 

 the branch or sub-kingdom of sponges ; the adult forms of which, by becoming fixed, 

 iiave undergone a retrograde development, the gastrula or larval forms showing a 

 promise of a state of development which the organism not only does not attain, 

 but from which it falls completely away in after life. Lankester regards the acepha- 

 lous molluscs, or bivalves, as having degenerated from a higher type of head-bearing 

 active creatures like the snails. The ascidians start in the same path of development 

 as the vertebrates, and at length fall back and lose nearly every trace of a vertebrate 

 alliance. 



Besides sub-kingdoms, classes, and orders of animals, we have 7iiinor groups which 

 are in their entirety examples of a backward development. Without doubt certain 

 human races, as the present descendants of the Indians of Central America, the mod- 

 ern Egyptians, " the heirs of the great Oriental monarchies of pre-Christian times," the 

 Fuegians, the Bushmen, and even the Australians, may be degenerate races. Lankes- 

 ter, in his book on " Degeneration," considers the causes of retrograde development 



