Chap. XIII. EMBRYOLOGY. 439 



these terms may be used literally ; and the wonderful 

 fact of the jaws, for instance, of a crab retaining nume- 

 rous characters, which they would probably have retained 

 through inheritance, if they had really been metamor- 

 phosed during a long course of descent from true legs, 

 or from some simple appendage, is explained. 



Embryology. — It has already been casually remarked 

 that certain organs in the individual, which when mature 

 become widely different and serve for different purposes, 

 are in the embryo exactly alike. The embryos, also, of 

 distinct animals within the same class are often strikingly 

 similar : a better proof of this cannot be given, than a 

 circumstance mentioned by Agassiz, namely, that having 

 forgotten to ticket the embryo of some vertebrate ani- 

 mal, he cannot now tell whether it be that of a mammal, 

 bird, or reptile. The vermiform larvae of moths, flies, 

 beetles, &c, resemble each other much more closely 

 than do the mature insects ; but in the case of larvse, 

 the embryos are active, and have been adapted for spe- 

 cial lines of life. A trace of the law of embryonic re- 

 semblance, sometimes lasts till a rather late age : thus 

 birds of the same genus, and of closely allied genera, 

 often resemble each other in their first and second 

 plumage; as we see in the spotted feathers in the 

 thrush group. In the cat tribe, most of the species are 

 striped or spotted in lines ; and stripes can be plainly 

 distinguished in the whelp of the lion. We occasion- 

 ally though rarely see something of this kind in plants : 

 thus the embryonic leaves of the ulex or furze, and the 

 first leaves of the phyllodineous acaceas, are pinnate or 

 divided like the ordinary leaves of the leguminosse. 



The points of structure, in which the embryos of 

 widely different animals of the same class resemble 

 each other, often have no direct relation to their condi- 



