Reproduction and Development. 57 



this material is of parental origin — is elaborated from the 

 nutriment absorbed and digested by the mother. 



Thus we see, in the higher types of life, parental sacrifice, 

 fosterage, and protection. For in the case of mammals 

 and many birds, especially those which are born in a 

 callow, half-fledged condition, even when the connection of 

 mother and offspring is severed, or the supplies of food-yolk 

 are exhausted, and the young are born or hatched, there is 

 still a more or less prolonged period during which the 

 weakly offspring are nourished by milk, by a secretion from 

 the crop (" pigeon's milk "), or by food-stuff brought with 

 assiduous care by the parents. There is a longer or 

 shorter period of fosterage and protection — longer in the 

 case of man than in that of any of the lower animals — ere 

 the offspring are fitted to fend for themselves in life's 

 struggle. 



And accompanying this parental sacrifice, first in 

 supplying food for embryonic development, and then in 

 affording fosterage and protection during the early stages 

 of growth, there is, as might well be supposed, a reduction 

 in the number of ova produced and of young brought forth 

 or hatched. Many of the lower organisms lay hundreds of 

 thousands of eggs, each of which produces a living active 

 embryo. The condor has but two downy fledglings in a 

 year; the gannet lays annually but a single egg; while 

 the elephant, in the hundred years of its life, brings forth 

 but half a dozen young. 



We shall have to consider by what means these opposite 

 tendencies (a tendency to produce enormous numbers of 

 tender, ill- equipped embryos, and a tendency to produce 

 few well-equipped offspring) have been emphasized. The 

 point now to be noted is that every organism, even the 

 slowest breeder that exists, produces more young than are 

 sufficient to keep up the numbers of the species. If every 

 pair of organisms gave birth to a similar pair, and if this 

 pair survived to do likewise, the number of individuals in 

 the species would have no tendency either to increase or 

 to diminish. But, as a matter of fact, animals actually do 



