THE DEVELOPMENT OF ORGANISMS 767 



This aspect of the problem is illustrated in the section on larvae; 

 a couple of illustrations must here suffice. The foal is much better 

 equipped at birth than the calf; it has, so to speak, a shorter infancy; 

 and this is adaptive to their differences in habit. For in natural 

 conditions the cow hides her calf in the thicket, whereas the foal 

 has to stumble along after its nomadic mother. Correlated with 

 this is the fact that the calf enjoys a prolonged meal, sucking to 

 repletion, while the foal is suckled hastily, but at frequent intervals. 

 Hence, too, the cow's udder is so large, and the mare's so small. 



The prolongation of ante-natal life and the suppression of youth- 

 ful stages must often have a high protective value, as is strikingly 

 illustrated by Peripatus. But another aspect is prominent in the 

 prolonged gestation of many mammals. As Robert Chambers pointed 

 out in his Vestiges of Creation, it may admit of the development of. 

 a larger brain before the time of critical testing begins. Thus the 

 centres of the cerebral cortex in particular may reach a high grade 

 of organisation before they begin to be flooded with sensory news 

 from the outer world or taxed by the requirements for control — 

 whether of eye- adjustment, of manipulative dexterity, or of agile 

 movements in general. 



But let the unifying idea stand out clearly that it is characteristic 

 of life-histories to show an ascending and descending curve, and 

 that different arcs on this curve or trajectory of life are shortened 

 or elongated adaptively in relation to particular circumstances of 

 life. Human life shows much the same series of phases upon its 

 life-curve as those mentioned at the outset; but of these we have 

 spoken briefly elsewhere. 



ALTERNATION OF GENERATIONS 



We see that the life-history of an organism, whether plant or 

 animal, is to be thought of as an ascending and descending curve, 

 with a summit that may be a peak or a plateau. From the vita 

 minima of the tender embryo the organism climbs through insur- 

 gent youth to maturity; and thence, sooner or later, it begins to 

 sink towards the vita minima of senescence. But the variations 

 on this general theme are many, and the trajectory of life must be 

 thought of as elastic in relation to seasons and circumstances. As 

 noticed above, there may be a long or a short embryonic develop- 

 ment, a long or a short youth, a long or a short adolescence, 

 and similarly for maturity and ageing. The elephant lives for 

 towards two years before it is born, and may not be mature till it is 

 40 years old. What a contrast to the entire life of a summer bee, 

 which may be all over in a couple of months or less. The eel has a 

 longish life of growing, say half a dozen years; but it seems always 



