1274 LIFE : OUTLINES OF GENERAL BIOLOGY 



multiplying recklessly. Another reason was that about that time 

 preventive medicine first began to make itself felt in reducing the 

 death rate. Furthermore, as "Clayhanger" brings out, it paid to 

 have many children when they could be sent early to the mine or 

 the factory. It paid to have a lot; and to that imworthy feeling 

 of the parents, that the more children they had the better, the 

 employers said, naturally enough from their point of view, 

 "Amen!" Those who have gone deeply into Natural History on this 

 line assure us that foxes quite approve of large families among 

 the rabbits ! 



So the population rose extraordinarily. And why did it fall? 

 That again is a very intricate and complex question with many 

 answers. One must take account, for instance, of the age composition 

 of the community, trying to find out how many wives under forty- 

 five there were during that period, and how many husbands under 

 fifty-five. In other words, one must inquire into the age of marriage 

 and the duration of marriage, and, if possible, into the tradition of 

 loyalty between husband and wife, and into the amount of illegiti- 

 macy, and into prudential and artificial birth control, and perhaps 

 there is something beyond all that — an obscure law which has not 

 yet been worked out for mankind. 



A fertilised egg cell, such as that which gives rise to a sea urchin, 

 divides into two, then into four, eigh*, sixteen, thirty-two, and so on. 

 It multiplies itself at first quite slowly, but after a while the multipli- 

 cation of cells is so quick that we cannot keep count of them. By 

 and by, however, the multiplication of cells begins to slow off — in 

 some organisms much more quickly than in others, and in some 

 organisms more slowly than in others. Among mammals the multipli- 

 cation of nerve-cells stops at birth. There is a decline till there is 

 no multiplication at all, a stationary condition of cells. Pearl's 

 population curve applies to the multiplication of cells in the building 

 of the body. Similarly, if yeast cells are placed in a suitable solution 

 they begin by multiplying rather slowly; soon they increase very 

 rapidly, then the rate decreases and the curve wanes away, till 

 finally the cells cease to multiply at all. The same curve is illustrated 

 by fruit flies battening on bananas in a jar; and Pearl's contention 

 is that the same curve applies to man as applies to yeast cells, or 

 to the hens in the poultry yard, or to the multiphcation of cells in 

 the development of an animal. In his book on the biology of popula 

 lion, Pearl goes very^ thoroughly into the interesting case of Algeria, 

 where there are censuses available for seventy years, and he has 

 found that the facts fit the curve most beautifully. Algeria is a 

 particularly good case to take because there is no complication of 

 the problem by birth control, and the public health service, though 

 not to be despised, has not begun to have any ver}^ powerful influ- 

 ence. It is probable then that this population curve expresses a 



