258 



GENERAL BIOLOGY 



are examples of this. Many others will be seen when a 

 variety of insect larvae illustrating types of metamorphosis 

 are before us in study 41. The immature stages of mayflies 

 offer a superb example. The adults are much alike in form 

 and in habits, but adult life is very brief and is concerned 

 only with reproduction. But the immature stages are 

 wonderfully unlike, being fitted for life in all sorts of waters. 

 They have specialized independently. 



The study of embryology furnished a new criterion of 

 homology. In addition to correspondence in parts and 



t+3 



**S 



Fig. 159. Wing of the nymph of a stonefly (Nemoura) showing tracheae, or 

 air tubes, of the wing, with the veins developing about them and be- 

 tween them; note the absence of tracheae from cross veins, and the lesser ' 

 angulation of the tracheae than of the veins. 



relations of parts in adults fundamental likeness has to be 

 judged also by correspondence in development. This is 

 indeed the sort of likeness that shows how deep seated is 

 the homology — the likeness that goes back to correspondence 

 in mode of origin. 



Embryology has also vastly extended the field in which 

 homologies may be determined ; for, owing to the primitive 

 conditions prevailing in the embryos, their parts may often 

 be readily compared and seen to be identical, when in adult 

 organs all likeness has disappeared. That the coracoid 

 process of the human scapula is in reality the equivalent of 



