THE INDIVIDUAL LIFE CYCLE 161 



— intermediate in some respects between worms and 

 insects — may carry its young ones before birth for 

 a whole year, for they must be born ready to fend 

 for themselves. Among wild horses the foal must 

 be able to run beside its mother soon after birth, 

 so the ante-natal embryonic period is drawn out to 

 eleven months. In the opossum the gestation is 

 sometimes telescoped down to a fortnight, for the 

 mother carries her baby after birth in a skin pocket 

 or attached to her tail. In bats, which are highly 

 specialised mammals, the ante-natal period is again 

 very short, for the young one hangs on to its mother 

 for a long time by toes and thumbs and mouth, and 

 has many a giddy journey through the air before it is 

 able to use its own wings. In birds the juvenile period 

 is often very short — mound-birds can actually fiy the 

 day they are hatched — for it is a precarious time. In 

 mammals the tendency is the other way, to lengthen 

 out the youthful period into a playing time, when this 

 can be done with safety. There are many interesting 

 facts of this sort, but all that we need here is an illus- 

 tration of the suggestive idea that one chapter in 

 the life-cycle may be lengthened out and another 

 telescoped down. We know in part how this can be 

 done, namely, hy variations in the rhythm of the 

 regulatory system — the organs of internal secretion. 

 It may also be effected in mankind by enlightened 

 adjustments of function and responsibility, and there 

 can be little doubt that true civilisation implies a 

 lengthening out of the youthful period. This is per- 



