408 OUTLINES OF EVOLUTIONAKY BIOLOGY 



the different parts of the animal body is affected by internal 

 secretions, or hormones, the products of various glands. Thus 

 we know that hypertrophy of the pituitary body in man may 

 lead to acromegaly, one of the symptoms of which is great 

 enlargement of certain parts. The most dreadful of all the 

 diseases to which human beings are subject, cancer, is essentially 

 due to an unrestrained multiplication of cells, and consequent 

 abnormal growth of tissue, which may very possibly be corre- 

 lated with the extent to which some specific internal secretion 

 is produced in the body. It also seems not unreasonable to 

 suppose that the growth of the body may be normally inhibited 

 or checked by specific secretions, and that in the absence of these 

 it may continue far beyond the ordinary limits. 



It is difficult to see any good reason why we should not apply 

 this principle to the race as well as to the individual, and, 

 paradoxical as it may appear, it even seems possible to explain 

 both the growth of the organism as a whole and that of its 

 various organs, beyond the limits of utility, as an indirect result 

 of natural selection. 



When a useful organ, such as the tusk of a wild boar, is first 

 beginning to develop, or to take on some new function for the 

 execution of which an increase in size will be advantageous, 

 natural selection will favour those individuals in which it grows 

 most rapidly and attains the largest size in the individual lifetime. 

 If growth is normally checked and controlled by some specific 

 secretion, or hormone, natural selection will favour those 

 individuals in which the glands which produce this secretion are 

 least developed, or at any rate least active. The process being 

 repeated from generation to generation these glands (whatever 

 may be their nature, and we use the term " gland "for any cell 

 or group of cells which produces a specific secretion, whether 

 recognizable as a distinct organ or not) may ultimately be 

 eliminated, or at any rate cease altogether to produce the par- 

 ticular hormone in question. Moreover, this elimination may 

 take place long before the organ whose growth is being favoured 

 by natural selection has reached the optimum size. When it 

 has reached this optimum it is certainly desirable that it should 

 grow no larger, but is there now any means by which further 

 growth can be checked ? The inhibiting hormone is no longer 

 produced ; the brake has been removed, and further growth may 

 be supposed to take place irrespective of utility, until, when 



