ZOOLOGY. 325 



lected one side of their subject, which, when properly considered, will 

 throw a great amount of new light on their investigations. Studying 

 animals, in general, it has been the habit to investigate them in their 

 full-grown condition, and scarcely ever to look back for their charac- 

 ters in earlier periods of life. We scarcely ever find, in a book on nat- 

 ural history, a hint as to the difference which exists in the young and 

 old. Perhaps in birds the color of the young may be noticed ; and it 

 is generally known that the young resemble the female more than the 

 male ; but as to precise investigation of the subject, we are deficient. 

 But if the early stages of life have been neglected, there is one period 

 in the history of animals which has been thoroughly investigated, for 

 the last twenty-five years, embryology. The changes which take 

 place within the egg itself, and which give rise to the new individual, 

 have been thoroughly examined ; but, after the formation of the new 

 being, the changes in its form which it passes through, up to its full- 

 grown condition, have been neglected. It had been his object to in- 

 vestigate this subject, because he had been struck with the deficiency 

 there is on this point in our works ; and, in making this investigation, 

 he had found that the young animals, in almost all classes, differ widely 

 from what they are in their full-grown condition. For instance, a 

 young bat, a young bird, or a young snake, at a certain period of 

 their growth within the egg, resemble each other so much that he 

 would defy the most able zoologist of our day to distinguish between 

 a robin and a bat, or between a robin and a snake. There is 

 something of high significance in this fact. There is something 

 common to all these. There is a thought behind these material phe- 

 nomena, which shows that they are all combined under one rule, and 

 that they only come, under different laws of development, to assume, 

 finally, different shapes, according to the object for which they were 

 introduced. 



There is a period of life, in which, whatever may be the final form 

 of their organs of locomotion, whatever may be the final difference 

 between the anterior and posterior extremities, vertebrated animals 

 have uniform legs in the shape of little paddles or fins. This is the 

 case with lizards as well as birds. A robin's wing and a robin's leg, 

 which are so different from a bat's wing and a bat's leg, do not essen- 

 tially differ when young from the leg and arm of a bat. Wherever 

 we observe combined fingers preserving this condition, we have a de- 

 cided indication that such animals rank lower in the group to which 

 they belong. This is all-important, as we are enabled at once to 

 group animals, which are otherwise allied, in a natural series, as soon 

 as we know whether they have combined or divided fingers. And 

 the degree of division to which the legs rise in their development is 

 a safe guide in our classification. Look, for instance, at the legs of 

 dogs and cats, in which the fingers are completely separated, and so 

 elongated that the animals walk naturally upon tiptoe, and compare 

 them with others, bears, for instance, which walk upon the whole sole 

 of the foot, and again with those of seals or bats, which remain united, 

 and constitute either fins or a wing. 



There are other reasons sufficient to convince us that the order of 



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