250 



THE ORIGIN AND EVOLUTION OF LIFE 



to the inheritance of bodily modifications caused by the neck- 

 stretching habit. Darwin attributed the lengthening of the 

 neck to the constant selection of individuals and races which 

 were born with the longest necks. Darwin was probably right. 

 This is an instance where length or shortness of neck is ob- 

 viously a selective survival 

 character in the struggle for 

 existence, because it directly 

 affects the food supply. 



But there are many other 

 changes of proportion in mam- 

 mals, which are not known to 

 have a selective survival value. 

 We may instance in man, for 

 example, the long head-form 

 (dolichocephaly) and the broad 

 head-form (brachycephaly) , or 

 the long-fingered form (dolicho- 

 dactyly) and the short-fingered 

 form (brachydactyly), which 

 have been interpreted as con- 

 genital characters appearing at 

 birth and tending to be transmitted to offspring. Brachy- 

 dactyly may be transmitted through several generations, but 

 until recently no one has suggested what may be its possible 

 cause. 



It has now been found^ that both the short-fingered con- 

 dition (brachydactyly) and the slender-fingered condition may 

 be induced during the lifetime of the individual in a previously 

 healthy and normal pair of hands by a diseased or injured con- 

 dition of the pituitary body at the base of the brain. If the 



1 Gushing, Harvey, 1911, pp. 253, 256. 



Fig. 119. Result of Removing the 

 Thyroid and Parathyroid Glands. 



(Right.) Normal sheep fourteen months 

 old. 



(Left.) A sheep of the same age from 

 which the thyroids and parathyroids 

 were removed twelve months previ- 

 ously. 



After Sutherland Simpson. 



