THE ARGUMENTS FROM EMBRYOLOGY. 465 



advantageous kind are going on elsewhere in the embryo a 

 useless part may remain long undiminished. 



Yet another cause operates, and perhaps cooperates. Em- 

 bryonic survival of an organ which has become functionless, 

 may readily happen if, during subsequent stages of develop- 

 ment, parts of it are utilized as parts of other organs. In the 

 words of Mr. J. T. Cunningham : 



"It seems to be a general fact that a structure which in meta- 

 morphosis disappears completely may easily be omitted altogether in 

 embryonic development, while one which is modified into something 

 else continues to pass more or less through its original larval condi- 

 tion." (Science Progress, July, 1897, p. 488.) 



One more factor of considerable importance should be 

 taken into account. A disused organ which entails evil 

 because construction of it involves needless cost, may entail 

 further evil by being in the way. This, it seems to me, is 

 the reason why the fore-limbs of snakes have disappeared from 

 their embryos. When the long-bodied lizard out of which 

 the ophidian type evolved, crept through stiff herbage, and 

 moved its head from side to side to find openings, there re- 

 sulted alternate bends of its body, which were the beginnings 

 of lateral undulations ; and we may easily see that in propor- 

 tion as it thus progressed by insinuating itself through inter- 

 stices, the fore-limbs, less and less used for walking, would 

 be more and more in the way; and the lengthening of the 

 body, increasing the undulatory motion and decreasing the 

 use of the fore-limbs, would eventually make them absolute 

 impediments. Hence besides the benefit in economy of con- 

 struction gained by embryos in which the fore-limbs were in 

 early stages a little less developed than usual, they would 

 gain an advantage by having, when mature, smaller fore- 

 limbs than usual, leading to greater facility of locomotion. 

 There would be a double set of influences causing, through 

 selection, a comparatively rapid decrease of these appendages. 

 And we may I think see also, on contemplating the kind of 

 movement, that the fore-limbs wo.uld be more in the wiy 

 than the hind limbs, which would consequently dwindle with 



