REPRODUCTION AND HEREDITY 275 



mysterious phenomena in the life of animals. To us it is simply 

 incomprehensible why many mud-flies which themselves feed on 

 honey but deposit their eggs in mud or even in manure-pits are 

 able to know that the conditions of life which are necessary and 

 pleasant for them are entirely unsuitable 

 for their offspring (fig. 66). That the 

 animals during the egg-laying process are 

 reminded of their own youth we are not 

 justified in assuming, for it would mean 

 giving too much credit to their intelli- 

 gence and to their memory. Phrases 

 such as ' innate impulse,' ' habit bred by 

 selection/ explain nothing and merely 

 serve as a cloak for ignorance. But the 

 case quoted here is only one out of 



innumerable instances of this mysterious FIG. 66. MUD-FLY (Eris- 



, tails tenax) WITH ITS LARV^: 



pbenomenon. WHICH LIVE IN MANURE-PITS. 



If there existed a degree in the incom- 

 prehensible I might quote as still less comprehensible the fact 

 that in preparing its cocoon a caterpillar does not make it to fit its 

 own size and form, but to fit the form of the emerging pupa, of 

 which it is impossible for the caterpillar to know anything from 

 experience. Similar unsolved questions are raised by the ingenious 

 and artistic construction of many of the nests of birds, and their 

 migration. How do the young birds find their long way to their 

 southern winter-quarters ? Who informs them that the autumnal 

 storms in the land of their birth announce a period of short rations 

 and famine ? Formerly it was believed that the inexperienced 

 young simply followed the parents. That may be true for many 

 species, but it has been demonstrated in others that the young 

 commence their flight before the old. We must, however, not lose 

 ourselves in such theoretical questions. All that I intended was 

 to show how many problems are still awaiting their solution, and 

 how unfounded is the fear that the rapid progress of science will 

 leave to future generations nothing to investigate. 



Species with a longer span of life are still better able to care 

 for their offspring. Among the most careless parents are most of 

 the Echinoderms which liberate their germ-cells into the sea, 

 where afterward impregnation and development take place. 



