THE DEVELOPMExNT OF ORGANISMS 793 



THE DURATION OF ANTE-NATAL LIFE 



One of the trends of organic evolution is towards viviparous birth. 

 That is to say, it has often been found advantageous that the off- 

 spring should develop within the mother, so that what is born is 

 already well advanced. One obvious advantage of this viviparity, 

 as against the egg-laying or oviparity that obtains in the vast 

 majority, is that the early development goes on in conditions of 

 relative safety, within the body of the mother. Laid or liberated 

 eggs are apt to be discovered by hungry eyes and eaten ; and among 

 land-animals they are often in danger of being dried-up. 



A correlated advantage of prolonged viviparity is that the young 

 creature is very well equipped when it is launched on the voyage of 

 life. In such a familiar case as sheep, the newly born lamb can 

 stagger after the ewe in a surprisingly short time, and everyone has 

 seen that the same is true of the foal which stumbles so quickly 

 after its mother. This is plainly of "survival value" in those gre- 

 garious mammals that are often on the move. A third advantage 

 of viviparity is seen in those types in which the unborn offspring 

 is a physiological partner with its mother, living for a prolonged 

 period in nutritive connection with her. This is pre-eminently true 

 of the Placental Mammals, in which there is a very intimate and 

 intricate union, by means of the placenta, between the mother and 

 the unborn young. Though no solids can pass through the placental 

 sponge from the one to the other, there is a diffusion of fluids and 

 gases between the two. The mother contributes to the blood of the 

 offspring (a) dissolved food-materials, (h) oxygen, and (c) some hor- 

 mones or chemical messengers. The offspring returns to the mother 

 (A) nitrogenous waste-products, (B) carbon dioxide, and, as some 

 say, (C) hormones which enable the mother to make the best of 

 her food. As (A) and (B) are poisonous, the advantage is very 

 obviously on the side of the offspring; yet possibly (C) the off- 

 spring helps the mother with a contribution of hormones. The 

 intimate interchange goes on during the whole period of gestation, 

 and the association well deserves to be called a symbiosis or vital 

 partnership. This is a truer view, we think, than is suggested by 

 the old description of the unborn mammalian offspring as a "foetal 

 parasite". 



VIVIPARITY. — Granting viviparity these advantages, we find 

 significance in the fact that it has been attained independently at 

 several different levels in the ascent of life, and has always been 

 rewarded with success for a time at least. One of the most archaic 

 of terrestrial animals, in some ways intermediate between Ringed 

 Worms (Annelids) and the insect-centipede stock (Tracheata- 



