278 REPORTS ON THE STATE OF SCIENCE. 



Is there any justification in recent developments of biological science 

 for the belief that a race of animals may acquire a momentum of the 

 kind referred to which may ultimately lead it to destruction ? Is there 

 some brake normally applied to the growth of organisms and organs, 

 and if so, are there occasions on which the brake may be removed, 

 leaving the organism to rush to destruction like a car running away 

 downhill? I hope to be able to show some ground for believing that 

 both these questions may be answered in the affirmative. 



It is, I think, now generally accepted by physiologists that the 

 growth of the different pails 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 leads to acromegaly, one of 

 the symptoms of which is great enlargement of certain parts. The 

 most dreadful of all diseases to which human beings are liable, cancer, 

 is essentially due to an unrestrained multiplication of cells, and conse- 

 quent 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 believing that, in the indi- 

 vidual, growth is normally inhibited or checked by specific secretions 

 and that in the absence of these it will continue far beyond the ordinary 

 limits. 



The question next arises, fan we apply this principle to the race 

 as well as to the individual ? I see no reason why we should not do 

 so, and, paradoxical as it may seem, I think we may be able to explain 

 the growth of the organism as a whole, and 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, natural selection 

 will favour those individuals in which the glands which produce this 

 secretion are least developed, or at any rate least active. This process 

 being repeated from generation to generation, these glands (whatever 

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

 group of cells which produces a specific secretion, whether recognisable 

 as a distinct organ or not) may ultimately be eliminated, or at any 

 rate cease altogether to produce the particular 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 there is no longer any means by which 

 the growth can be checked : the inhibiting hormone is no longer pro- 

 duced, the brake has been removed, and further growth will take place 

 irrespective of utility, until, when the size of the organ gets too great 

 to be compatible with the well-being of the individual, natural selection 

 again steps in and eliminates the race. The same argument of course 

 applies to the size of the body as a whole, as well as to that of its 

 constituent organs. Is it not possible that, the normal checks to growth 



