46 HELEN DEAN KING 



Hatai ('07), Osborne and Mendel ('14; '15; '16) and Stewart 

 ('16) have shown that the growth of the albino rat can be in- 

 hibited for varying periods, either by starvation or by improper 

 food, and that there is at once a resumption of growth when a 

 return is made to a normal diet ; the animals eventually reaching 

 the size of the controls or even surpassing them in body weight 

 at corresponding ages. 



Although, as a rule, adult rats increase in body weight very 

 slowly and may even remain at practically the same body weight 

 for several successive months, this slowing up of the growth 

 process is apparently not due to an exhaustion of the growth 

 capacity. The extensive experiments of Osborne and Mendel 

 show that the capacity to grow can be retained and exhibited at 

 periods far beyond the age at which growth ordinarily ceases, 

 and their work points to the conclusion that in the rat "the ca- 

 pacity to grow is only lost by the exercise of this fundamental 

 property of animal organisms." What is true for the individual 

 may also be true for the race. The capacity to grow is seem- 

 ingly so essential a part of the organism that this power is re- 

 tained through several successive generations in which it is not 

 exercised to its full extent. In this series of experiments, as far 

 as is known, not a single individual of the many hundreds that 

 were reared in the first four generations attained a body size 

 that equaled the norm for the adult albino rat. Yet even after 

 this long period of time the growth impulse in all individuals 

 at once responded to the stimulus of adequate nutrition, and 

 only two generations were required to effect a return to normal 

 body size. 



That favorable nutritive conditions had produced a parallel 

 modification of the soma and of the germplasm might be a sat- 

 isfactory explanation for the appearance of the exceptionally 

 large individuals in the seventh to the ninth inbred generations 

 were it not for the fact that this increase in the body size of the 

 individuals was temporary, lasting through these generations 

 only. It seems more probable that favorable nutritive condi- 

 tions, following a period of semi-starvation, greatly increased 

 metabolic activity and so stimulated the growth impulse that the 



