INTRODUCTION. xvii 



Iloiiiology may be general or special; the latter limited, for the most part, to 

 animals of the same sub-kingdom. It is well to use words which will express our 

 meaning exactly, and hence a general homology may be indicated by the word 

 isogeny, indicating a general similarity of origin; thus, the nervous system of 

 worms, arthropods, molluscs, and vertebrates are isogenous, all being derivations 

 of the epiblast. The term homology should be restricted to those cases where the 

 correspondence, part for part, is more exact. Thus, the brain of fishes and that of 

 man are not only isogenous, but homologous. 



The Scale of Pekfection in Organs and in Animals. 

 The history of the rise and progress of the human arm, possibly, as some claim, 

 from an organ like the fin of a shark, a boneless, flabby limb ; how it gradually, by 

 adaptation, became like the differentiated fore-leg of a salamander, then became adapted 

 for a climbing, arboreal use, and finally became, next to his brain, the most distinctive 

 organ in man, — the history of the successive steps in this rise in the scale of perfec- 

 tion would throw light on the general subject of the gradual perfection of organs and 

 organisms. On the other hand, the hinder extremities or legs of man have not been 

 equally perfected. As Cope has remarked : " He is plantigrade, has five toes, separate 

 carpals and tarsals, a short heel, rather flat astragalus, and neither hoofs nor claws, but 

 something between the two." Man's limbs are not so extremely specialized as those 

 of the horse, which is digitigrade, walking only on four toes, one to each foot. INIan's 

 stomach is simple, not four-chambered, as in the ox. Thus one organ, or one set of 

 organs, may in man attain the highest grade in the scale of perfection, while others 

 maybe comparatively low in form and function. Animals acquire, so to speak, their 

 form by the acceleration in the growth of certain parts, which involves a retardation 

 in the development of others ; it is by the unequal development of the different parts 

 that the fish has become adapted to its life in the water, that the bird becomes fitted 

 for its aerial existence, and that moles can burrow, and monkeys are enabled to climb. 

 The scale of perfection, as applied to organs, is a relative one ; those in each animal 

 are most perfect which are best adapted to subserve the requirements of that creature. 

 Man's brain is on the whole the most perfect of all organs, and it enables him to regu- 

 late the movements of his limbs and other organs in a manner alone characteristic of 

 an intellectual, reasoning, speaking, spiritual being. 



Generalized and Specialized Types. 

 A large proportion of the higlier classes of animals now living are more or less 

 specialized ; they stand at or near the head of a series of forms which have become 

 extinct, and which were much less specialized. For example, there are now living 

 nearly ten thousand species of bony fishes, while the remains of only about twenty 

 species have been found in the cretaceous formation. The earlier types of fishes were 

 generalized or composite in their structure, presenting, besides the cartilaginous 

 skeleton, a feature occurring in the embryos of lung fishes, characteristics which place 

 them above the bony fishes. Among fishes, the lung fishes or Dipnoi, are the clearest 

 example of a generalized type ; they have a notocord, in which respect they resemble 

 the lancelet and lamprey ; while they possess one or two lungs, in which respect they 

 resemble the salamanders or batrachians ; thus in some features they are lower, and 

 in others higher, than any of the bony fishes. There is a strange mixture of characters 

 in these composite animals, and the living forms may be regarded as old-fashioned, 



