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THE POPULAR SCIENCE MONTHLY. 



types, as a crab or a lobster, with limbs some very small and some 

 very large. How came this contrast to arise in the course of evo- 

 lution, if there was the equality of variation supposed ? 



But now let us narrow the meaning of the phrase still further ; 

 giving it a more favorable interpretation. Instead of considering 

 separate limbs as co-operative, let us consider the component parts 

 of the same limb as co-operative, and ask what would result from 

 varying together. It. would in that case happen that, though the 

 fore and hind limbs of a mammal might become different in their 

 sizes, they would not become different in their structures. If so, 

 how have there arisen the unlikeness between the hind legs of the 

 kangaroo and those of the elephant ? Or if this comparison is 

 objected to, because the creatures belong to the widely different 

 divisions of implacental and placental mammals, take the cases of 

 the rabbit and the elephant, both belonging to the last division. 

 On the hypothesis of evolution these are both derived from the 

 same original form, but the proportions of the parts have become 

 so widely unlike that the corresponding joints are scarcely recog- 

 nized as such by the unobservant: at what seem corresponding 

 places the legs bend in opposite ways. Equally marked, or more 

 marked, is the parallel fact among the Articulata. Take that limb 

 of the lobster which bears the claw and compare it with the cor- 

 responding limb in an inferior articulate animal, or the corre- 

 sponding limb of its near ally, the crayfish, and it becomes obvious 

 that the component segments of the limb have come to bear to one 

 another in the one case proportions immensely different from those 

 they bear in the other case. Undeniably, then, on contemplating 

 the general facts of organic structure, we see that the concomitant 

 variations in the parts of limbs have not been of a kind to produce 

 equal amounts of change in them, but quite the opposite — have 

 been everywhere producing inequalities. Moreover, we are re- 

 minded that this production of inequalities among co-operative 

 parts, is an essential principle of development. Had it not been 

 so, there could not have been that progress from homogeneity of 

 structure to heterogeneity of structure which constitutes evolution. 



We pass now to the second supposition : — that the variations 

 in co-operative parts occur irregularly, or in such independent 

 ways that they bear no definite relations to one another — miscel- 

 laneously, let us say. This is the supposition which best corre- 

 sponds with the facts. Glances at the faces around yield conspic- 

 uous proofs. Many of the muscles of the face and some of the 

 bones, are distinctly co-operative ; and these respectively vary in 

 such ways as to produce in each person a different combination. 

 What we see in the face we have reason to believe holds in the 

 limbs as in all other parts. Indeed, it needs but to compare people 

 whose arms are of the same lengths, and observe how stumpy are 



