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THE CONTINUITY OF THE RACE 



Methods of ensuring fertilization. With more than a half million 

 bisexual species, variously adapted to live in so many diverse types of 

 situations, we cannot hope to survey all the interesting behaviors that 

 have been developed to ensure fertilization of the eggs. We can, how- 

 ever, classify practically all known types of habits into four groups that, 

 although they somewhat overlap, illustrate something of the variety and 

 the range in complexity and efficiency of these methods. The most funda- 

 mental distinction is that between external and internal fertilization. 



External fertilization without amplexus is correlated with an aquatic 

 (usually marine) habitat, a more or less sedentary type of life, and is 

 usually associated with the congregating of large numbers of male and 

 female individuals into a comparatively small area during the breeding 



Fig. 15.8. A pair of toads, Bufo terrestris 'americanus, in amplexus. The male holds the 

 female tightly, with his thumbs pressed deep into her sides. As the eggs are laid sperma- 

 tozoa are poured over them by the male. (Photo by Prof. S. C. Bishop.) 



season. There the females pour out their eggs into the sea water, and 

 the males liberate tremendous numbers of sperms. The eggs drift about 

 and diffuse out some substance that serves as an attraction to the sperma- 

 tozoa of their own species. Sperms that chance to swim close enough to 

 the egg to encounter this substance are attracted and guided to the egg 

 and fertilize it. The approach of the sperm close enough to the egg to be 

 attracted to it is a matter of chance, but chance that is very much in- 

 creased by the proximity of the males and females and by the tremendous 

 number of eggs and sperms that are produced. 



External fertilization with amplexus 1 shows a much more elaborate 

 and efficient development of breeding habits. Here, too, an aquatic situa- 



1 Literally, amplexus means "embrace" and is more correctly applied to the clasp- 

 ing of the female frog or toad in the arms of the male; however, its extension to include 

 the behavior of many fishes during fertilization is physiologically if not morpho- 

 logically justified. 



