ORGANIC EVOLUTION 225 



nearly all mammals there are seven cervical vetebrae, 

 whether the neck be long as a giraffe's or short as a mole's. 

 The foremost is the atlas vertebra, and bears up the skull; 

 the second is the axis vertebra, about which the atlas swmgs; 

 the other five, although less differentiated, are equally- 

 constant in position and relations, and we can not doubt but 

 that these seven are i-dentical. The fore limbs of vertebrates 

 are sufficiently unlike in superficial appearance; we know 

 them as legs in most quadrupeds, as flippers in seals, as 

 wings in birds and bats, and as arms in ourselves; but 

 when we examine their structure we find they are built on a 

 common plan (fig. iii), and therefore, hom.ologous. The 

 recognition of homologies often calls for the utmost care in 

 comparison of organs and for discriminating judgment of a 

 high order. It was a dictum of the elder Agassiz that the 

 education of a naturalist consists in learning how to compare. 



There is beside this correspondence of parts between 

 different organisms, a similar correspondence betw-een parts 

 that are serially repeated in a single organism. This is 

 called serial homology. It is well represented in the repeti- 

 tion of parts, segment by segment in the earthworm. 



The student in this course has already had in Chapter I, a 

 little practice in identifying homologous parts; first, in 

 flowers (pistils, stamens, corolla, etc.), and later in the parts 

 of the body of insects. A special study of this matter is 

 given here with material more available for critical examina- 

 tion. 



The veins in the wings of insects. 



The veins that constitute the supporting frame work of 

 an insect wing may bear the following names and designa- 

 tions: 



Costa (O Subcosta (5c) Radius {R) 

 Media (M) Cubitus {Cu) Anal veins (.4) 



