LIFE AND THE CELL ^^ 



might be like if evolution had continued to go on without the development 

 of sex. Certainly, the higher animals, if there were any, would be very 

 different from what they now are structurally, physiologically, emotion- 

 ally and intellectually. The reader may try to imagine what sort of crea- 

 tures they would be. His guess would be as good as that of the professional 

 biologist. 



We shall not discuss the problem of the biological significance of sex 

 further than to state that its great importance in evolution is attested by the 

 fact that only very primitive organisms were evolved until the advent 

 of sexual reproduction. Then evolution took a spurt upward. The most 

 primitive manifestation of sexual reproduction is the conjugation of two 

 similar simple organisms. Both the nuclei and the surrounding protoplasm 

 of the conjugants fuse to become one flesh, after which often following a 

 resting stage, multiplication by fission goes on as before. At first there is no 

 clear distinction of male and female, but in many one-celled organisms, 

 plants as well as animals, the conjugating individuals are differentiated into 

 a large, relatively immobile female cell and a much smaller, actively swim- 

 ming male cell. This differentiation parallels the differences between the 

 ovum, or egg cell, and the spermatozoon of the higher animals. 



In all the multicellular animals the sex cells are sharply differentiated 

 into eggs and sperm, but in more primitive groups, such as sponges, corals, 

 jelly fish and many worms and molluscs, sexual reproduction is usually 

 accomplished quite simply by discharging the eggs and sperm into the 

 water and leaving their union to chance. In all but the simplest of the multi- 

 cellular animals the sex cells are produced in specialized organs, often pro- 

 vided with ducts for their transfer to the outside. But sexual reproduction 

 does not involve elaborate behavior or many accessory structures until 

 the development of internal fertilization. This step is one of tremendous 

 importance for further evolution. We see it foreshadowed, as it were, in 

 certain groups of animals in which the fertilization of eggs still occurs 

 outside the body, by the development of instincts that bring about a close 

 association of the sexes during the breeding season. During the period of 

 egg laying in fishes, for instance, the female is closely followed by the 

 male, who frequently rubs against her body and discharges his milt, or 

 sperm, over the eggs as soon as they are extruded. In the breeding season 

 the males of many species develop brighter colors and, sometimes, small 

 bodily protuberances and other structures associated directly or indirectly 

 with the function of mating. These modifications are not, as a rule, exten- 

 sive. In frogs, toads and some other amphibians a closer association of the 

 sexes is secured by the clasping instinct of the male. As in fishes, the dis- 

 charge of the eggs from the female prompts the simultaneous discharge 

 of the sperm from her male companion, the eggs being fertilized in the 

 water by the sperm which penetrate their jelly-hke covering. That such 

 mating habits probably led to internal fertilization is indicated by the fact 



