392 GREAT STEPS IN ORGANIC EVOLUTION 



cock and peahen, ruff and reeve, stag and hind, man and 

 woman. Sexual reproduction meant much in immediate 

 .reward. It meant a more economical means of continuing 

 the race; it meant a device for securing the persistence 

 of a successful constitution and for screening the offspring 

 from disadvantageous dints made on the parent's body; it 

 meant more opportunities for re-arrangements of the hered- 

 itary items at the beginning of each new life. But the sep- 

 aration of sperm-producers or males and egg-producers or 

 females, differing deeply in constitution, would also tend 

 to increase the possible range of cross-fertilisation, which 

 is often advantageous, and would permit of a very profitable 

 division of labour between the two parents in their relations 

 to the offspring. For the masculine constitution has rarely 

 proved adaptable to mothering. But at a great distance the 

 divergence attained another justification, for sex-dimorphism 

 afforded a basis for love, becoming a liberator and an edu- 

 cator of emotions which have enriched and ennobled the 

 lives of many creatures. It is surely a fact of encourage- 

 ment to man that there are in the evolution of sex many 

 instances of the sublimation of what was, to start with, 

 somewhat rough and crude, into what, in some birds for 

 instance, it is difficult not to regard as a fine affection. 



7. Progressive Differentiations and Integrations. 



A multitude of evolutionary steps of great interest must 

 be summed up in the phrase progressive differentiation. Tis- 

 sues begin in sponges, organs in the stinging animals or 

 Crelenterates. As we ascend the series we see organ added 

 to organ in a way that suggests inexhaustible resources. 



The radial symmetry of sponge and zoophyte, jellyfish 

 and coral, well enough suited for a sedentary or for a drift- 



