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 swings; 

 the other five, although less differentiated, are equally 

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

 that these seven are identical. 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. in), and therefore, homologous. 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 between 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 (C] Subcosta (Sc] Radius (R) 

 Media (M) Cubitus (Cu) Anal veins (A) 



