128 



THE INrLCENCB OF INANIMATE SUEEODNDINGS. 



to absorb their whole vital energy ; thus an animal which was 

 forced by such external conditions as we have been supposing 

 to lay its eggs while still in the larva stage must always, if it 

 dies soon after from exhaustion, produce only such progeny as 

 are sexually mature in the larva-form so long as the conditions 

 remain the same — in this instance, namely, a low optimum of 

 temperature for the maturation of the eggs. 



We will here interrupt this chain of hypotheses, since they 

 are introduced merely to explain the possible origin of such an ■ 

 apparently paradoxical sexual larva-form. 



Fig. 33,— Apus. 



We must, however, briefly touch on another circumstance 

 which plainly demonstrates the direct influence which may be 

 ■exercised by variations of temperature on the gi'owth and de- 

 velopment of individuals. 



The general efiect of a rise of temperature on the develop- 

 ment of the embryo and young animal is well known. We know 

 that a hen's egg requires for its quickest normal development a 

 warmth kept as constantly as possible at 40° centigrade ; at a 

 lower temperature the development will be delayed, while at the 

 ordinary temperature of a sitting-room it ceases alt(^ether. The 

 eggs of certain Crustacea, on the other hand, as Apus {Lepidurus, 



