6o HOW ANIMALS DEVELOP 



development within a womb is a fairly new idea in 

 evolution ; mammalian embryos are not yet perfectly 

 adjusted so as to take advantage immediately of the 

 favourable condition which a womb provides. 



The length of time between conception and birth 

 is different in different mammals. On the whole, the 

 bigger the animal the longer the time, but there is 

 not a strict proportionality; the smaller ones take 

 longer than would be expected, and so does man. 

 In elephants the time is about 620 days, during 

 which time new elephant tissue is formed at an 

 average rate of 14 lbs, per day, while the mouse, 

 which is only one-quarter-millionth as big, takes 

 21 days to produce an infant, making new "mouse'* 

 at the rate of only a fiftieth of an ounce per day. 

 Bigger animals also live longer than smaller ones, 

 and the whole tempo of their lives is slacker. Perhaps 

 each animal has its own apprehension of time, so 

 that for instance a mouse feels a minute of man-time 

 as a whole quarter-of-an-hour of mouse-time, while 

 to the elephant it is only a few seconds of elephant- 

 time. 



