1867.] 277 [Wynan. 



with the phalanges, that is, in having the proximal epiphysis the last 

 to unite with the shaft instead of the distal. Still the preponderance 

 of facts is the other way, and, if we adopt the idea of symmetry, we 

 must rest content with the assumption that the thumb with its two 

 phalanges is the homotype of the outer toe with its three phalanges. 



In the preceding pages the object has been to set forth some rea- 

 sons for studying the fore and hind parts of vertebrated animals, but 

 more especially their limbs, as if these parts and limbs were con- 

 structed not only after one and the same type, but in a symmetrical 

 manner. They would repeat each other exactly in an ideal animal, 

 just as the right and left parts do in the actual. In the actual animal 

 the fore and hind parts are so modified as to adapt them to special 

 conditions of existence, and as the conditions fulfilled by the two 

 kinds of limbs are generally different, the limbs take on different de- 

 grees and phases of development. Right and left parts repeat each 

 other almost exactly, because their conditions are the same, though 

 even these, as we have already seen, may sometimes vary, and then 

 we have a diversity in their development. In fore and hind limbs 

 diversity is the rule, while in right and left it is the exception. Nev- 

 ertheless, as we go back to the early stages of embryonic life, the 

 symmetry and equality of fore and hind parts becomes nearly exact, 

 however much they may vary in the adult. 



We have not forgotten that in attempts like the present, compari- 

 sons should be made, not only between the bones of the limbs, but also 

 between the muscles, nerves and vessels. We have confined our re- 

 marks chiefly to the bones, because their homologies are the most 

 accurately determined. The attempts hitherto made for the determi- 

 nation of the homologies of the other parts, have been far less satisfac- 

 tory. If a serious objection can be brought against the mode we have 

 adopted of viewing the bones, far more serious objections can be 

 brought against such a method of viewing the other structures. If 

 the method fails in the skeleton, it will certainly fail elsewhere. On 

 the other hand, if antero-posterior symmetry can be shown to exist in 

 the bones, then we can feel some confidence, that whatever the diffi- 

 culties at present may be with regard to the muscles, nerves and ves- 

 sels, they will sooner or later be overcome. We may go still further, 

 and assert that if the idea of fore and hind symmetry enters into the 

 composition of animal structures at all, it will be traced not only in 

 the limbs, but in all the great systems of organs. Unity of plan in 

 the structure and composition of animals is much more likely to prove 

 true than diversity. 



Attempts have been made to construct an ideal skeleton, an "arche- 

 type" which is presumed to contain all the essential elenn.'ntary parts 

 of a vertebrated skeleton ; these parts nowhere so developed as to 



