CiiAr. XIII. AND EMBRYOLOGY. 395 



of a lly, namely, the Cecidomyia, producing ascxually other 

 and similar larvie. 



It has already been stated that various parts and organs in the 

 same individual are exactly like each other during an early em- 

 bryonic period, but in the adult state become widely different 

 and serve for widely-different puqioses. So again it has been 

 slated that the embryos of the most distinct species within the 

 same class are generally closely similar, but become when fully 

 developed widely dissimilar. A better proof of this latter fact 

 cannot be given than that by Von 15aer, namely, that " the em- 

 bryos of mammalia, of birds, lizards, and snakes, probably also 

 of eheloniii, are in their earliest states exceedingly like one 

 another, both as a whole and in the mode of development of 

 their parts ; so niuch so, in fact, that we can often distinguish 

 the emlnyos only by their size. In my possession are two 

 little embryos in sj)irit, whose names I have omitted to attach, 

 and at ]iresent I am quite unable to say to what class they be- 

 long. They may be lizards or small birds, or very young- 

 mammalia, so complete is the similarity in the mode of forma- 

 tion of the head and trunk in these animals. The extremities, 

 however, are still absent in these embryos. But even if they 

 had existed in the earliest stage of their development we 

 should learn nothing, for the feet of lizards and mammals, the 

 wings and feet of birds, no less than the hands and feet of 

 man, all arise from the same fundamental form." The vermi- 

 form larva? of moths, flies, beetles, etc., generally resemble each 

 other much more closely than do the mature insects ; but in 

 these cases the embryos arc active, and from having been 

 adapted for special lines of life sometimes differ much from 

 each other. A trace of the law of embryonic resemblance oc- 

 casionally lasts till a rather late age : thus birds of the same 

 genus, and of closely-allied genera, often resemble each other 

 in their inunature plumage ; as Ave see in the spgtted feathers 

 in the young of the thrush group. In the cat tribe, most of 

 the species arc striped or spotted in lines; and stripes or spots 

 can be plainly distinguished in the whelp of the lion and the 

 puma. We occasionally though rarely sec something of this 

 kind in ])lants; thus the first leaves of the ulex or furze, and 

 tlu! lirst leaves of the j)hyllodincous acacias, are pinnate or di- 

 vided lik<^ the ordinary leaves of the leguminosa\ 



The points of structure, in which the embryos of widely- 

 different animals within the same class rcsend^le each other, 

 often have no tlirect relation to their conditions of existence. 



