70 The Outline of Science 



implied in the formation of two kinds of germ-cells the ovum 

 or egg-cell, with a considerable amount of building material and 

 often with a legacy of nutritive yolk; the spermatozoon or sperm- 

 cell, adapted to move in fluids and to find the ovum from a 

 distance, thus securing change-provoking cross-fertilisation. 



* 

 The Evolution of Sex 



Another of the great steps in organic evolution was the 

 differentiation of two different physiological types, the male or 

 sperm-producer and the female or egg-producer. It seems to be 

 a deep-seated difference in constitution, which leads one egg to 

 develop into a male, and another, lying beside it in the nest, into 

 a female. In the case of pigeons it seems almost certain, from the 

 work of Professor Oscar Riddle, that there are two kinds of egg, 

 a male-producing egg and a female-producing egg, which differ 

 in their yolk-forming and other physiological characters. 



In sea-urchins we often find two creatures superficially in- 

 distinguishable, but the one is a female with large ovaries and the 

 other is a male with equally large testes. Here the physiological 

 difference does not affect the body as a whole, but the repro- 

 ductive organs or gonads only, though more intimate physiology 

 would doubtless discover differences in the blood or in the chemical 

 routine (metabolism). In a large number of cases, however, 

 there are marked superficial differences between the sexes, and 

 everyone is familiar with such contrasts as peacock and peahen, 

 stag and hind. In such cases the physiological difference between, 

 the sperm-producer and the ovum-producer, for this is the 

 essential difference, saturates through the body and expresses 

 itself in masculine and feminine structures and modes of behav- 

 iour. The expression of the masculine and feminine characters is 

 in some cases under the control of hormones or chemical mes- 

 sengers which are carried by the blood from the reproductive 

 organs throughout the body, and pull the trigger which brings 



