VERTEBRATES: BACKBONE AND BRAIN 89 



In these closely associated systems of blood-vessels, 

 nutriment and oxygen diffuse from the blood of the 

 mother into that of the embryo, and thus rapid growth 

 is assured. The importance and far-reaching effect of 

 this new modification in the old reproductive system 

 cannot be over-estimated. The internal intra-uterine 

 development of the young, and the mammalian habit 

 of suckling them, far more than any other factors, have 

 made man what he is. Some explanation must be 

 sought for such a fact. 



We have already seen that any animal devotes to 

 reproduction the balance between income and expendi- 

 ture of nutriment. Now, the digestive system is here 

 well developed, and the income is large. But we have 

 already noticed that, as animals grow larger, the ratio 

 between the digestive surface and the mass to be sup- 

 ported grows continually smaller. On account of size 

 alone the mammal has but a small balance. But the 

 amount of expenditure is proportional to the mass and 

 activity of the muscular and nervous systems. And 

 the mammal is, and from the beginning had to be, an 

 exceedingly active, energetic, and nervous animal. The 

 income has increased, but the expenses have far out- 

 run the increase. The mammal can devote but little 

 to reproduction. 



Moreover, it requires a large amount of material to 

 form a mammalian egg, such as that of the monotreme. 

 It requires indefinitely more nutriment to build a 

 mammal than a worm, for the former is not only 

 larger and more perfect at birth ; it is also vastly more 

 complicated. The embryonic journey has, so to speak, 

 lengthened out immensely. One monotreme egg rep- 

 resents more economy and saving than a thousand 



