408 OUTLINES OF EVOLUTIONARY BIOLOGY 



the different parts of the animal body is controlled by internal 

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

 we know that disease 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 unre- 

 strained multiplication of cells, and consequent abnormal growth 

 of tissue, which may very possibly be correlated with the extent 

 to which some specific controlling secretion is produced in the 

 body. In short, we are justified in supposing that in the individual 

 growth 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 



