No. 614] PHYSIOLOGICAL PROBLEMS 



141 



the protein content of the dietary below the minimum re- 

 quired for growth; (4) by furnishing as the exclusive 

 source of nitrogenous intake proteins which lack some 

 amino-acid group indispensable to growth. 



Thus the animals were maintained at practically the 

 same weight and they retained their power to grow long 

 past the age at which growth normally ceases (335 days) 

 and for periods equal to half the normal life of the spe- 

 cies, which is 1,000 days. 



Wodsedalek (T7) has shown that certain tenebrionid 

 larvae can not only be maintained, but may be reduced 

 from half-grown to hatching size several times by re- 

 peated starving and feeding. This seems to leave little 

 doubt as to the existence of a threshold of development 

 for food. 



(h) Definite Amount of Development 

 Krogh has shown that the total amount of carbon diox- 

 ide given off by pupa 1 of Tvuchrio molitor is the same for 

 all temperatures, showing that there is a definite amount 

 of development to be attained. The rate appears to be 

 different for different species where no considerable dif- 

 ference in the total for passing the stage in question is to 

 be expected, as in the case of fishes (see graphs by 

 Krogh). Thus, difference in velocity and increase in 

 velocity at different temperatures and moistures, etc., 

 have an important hearing on the variable or unequal sea- 

 sonal appearance of the different species. The accelera- 

 tion of development under conditions of factors near the 

 threshold is a further consideration (for a noteworthy 

 instance see Bachmetjew's ('07) retabulation of Mer- 

 rifeld's ('00) data) which leads to non-coincident appear- 

 ance and peculiar modification of normal sequence in ab- 

 normal seasons. 



It appears that the chief reason that there are not more 

 generations in an annual cycle in the case of spiders or 

 other animals is that the amount of energy which must 

 be expended and the velocity of development are such 



