88 Walter Shipleji : 



Veterinary Anatomy, jjage 33, records similar abnormalities. This 

 change of position of the costal process is clearly compensatory, and 

 it is probably due to the impulses that cause these horses to lead 

 off the gallop from the same foot. 



The extinction of the costal processes from the seventh vertebra by 

 the flexion of the neck seems to clear up the point which puzzled 

 Darwin, how structure may become extinct. The costal processes of 

 the seventh neck bone disappear before hostile conditions that are 

 sufficiently hostile to suppress them to extinction. We have evidence 

 that the costal processes of the seventh vertebra are suppressed to 

 extinction by the flexion of the neck ; and this fact shows a reason for 

 the extinction of definite structure and it goes to prove that structures 

 and species are controlled by the same law. If it be admitted that 

 species and structures are controlled by the same law, it follows that 

 cervical ribs in mammals represent a new type of rib situated in the 

 same positions as ribs that have suffered extinction ; and, therefore, 

 they are not atavistic structures but new developments. 



If it be admitted that niannimlian cervical ril)s are atavisms, then 

 it must be admitted that extinction of rib-structure is a throw-back to 

 the invertebrates. 



A brief account of some of the hostile influences that have assailed 

 ceiwical ribs, reduced them to vestiges, and finally caused their ex 

 tinction will be dealt with in the body of this paper. 



Impulses. 



Smith- Woodward writes thus (Ann. Nat. Hist, xviii., 190G, page 

 312): — 



" Throughout the evolution of the organic world there has been a 

 succession of impulses, each introducing not only a higher state of 

 life, but also fixing some essential characters that have been variable 

 in the grade immediately below." 



From this quotation it does not sccui clear what an impulse is. 

 Does it represent the action of environment on the organism, or the 

 reaction of living tissues to external conditions from which results 

 the generation of impulses] These two things are \ery ditiereiii ; 

 one ignores the re-aetioii of tissues and the other regards it as an 

 essential factor. In dealing with neck tissues it seems impossible to 

 ignore the biological factor and to cx|ilain the evolution of the neck 

 by physical conditions alone. Vov instance, there is a general belief 

 that " continued pressure causes atrophy, and inttiriuitleiit pressure 

 hypertrophy,' and yet the necks of porpoises and whales that are 

 submitted to intermittent pressure show atrophic changes. ThL\se 

 neck tissues react to the impulses which the biological factor in 

 reaction with external conditions generates. 



