THE BREEDING BEHAVIOR OF ANIMALS 



185 



No Evolutionary Sequence. — Since some of these types of breeding 

 behavior are plainly much more specialized than others, one might be 

 tempted to suppose that they exhibit some sort of evolutionary sequence. 

 That is, it might be thought that the simpler habits would be employed by 

 the more primitive groups of animals, while the complicated methods 

 would be adopted by the higher forms. Such appears not to be the case, 

 however, either as to assurance of fertilization or as to place of develop- 

 ment. Thus, copulation, which is a specialized habit, is employed by 

 some parasitic worms, some snails, the insects, reptiles, birds, and mam- 

 mals. These groups are so diverse in structure that it is impossible to 

 regard them as all primitive or all 

 highly developed. Furthermore, 

 most of the fishes and amphibia 

 use either external fertilization with 

 clasping or internal fertilization 

 without clasping, while some mem- 

 bers of each of these groups employ 

 copulation. In general, the same 

 breeding habits may occur in 

 animals of widely different groups, 

 and animals of the same group often 

 have very different habits. The 

 principal generalization concerning 

 fertilization is that among aquatic 

 or amphibious forms the habit pre- 

 vails of depositing the spermatozoa 

 and eggs freely in the water or 

 in immediate proximity to each 

 other, or of depositing the sperma- 

 tozoa so that they can be secured 

 later by the female; while in 

 the groups composed mostly of land forms the habit of introducing them 

 into the body of the female predominates. The latter method is essen- 

 tial to most land forms, since air is fatal to the delicate sexual cells, 

 whereas in aquatic forms the eggs (at least after fertilization) can endure 

 the water for a prolonged period. 



In the method of bearing the young, also, there is no evolutionary 

 sequence. Oviparity and viviparity are found in the vertebrates and the 

 invertebrates. Certain conditions of reproduction itself, however, make 

 one generalization possible. The forms in which the eggs are fertilized 

 outside the body of the mother are necessarily oviparous; and it is only 

 among forms mth internal fertilization that viviparity, ovoviviparity, 

 and the laying of fertilized eggs can occur. As a result, viviparity, 



Fig. 154. — Embryo sharks of a vivip- 

 arous species, Mustelus mustelus (Lin- 

 naeus), attached to the wall of the uterus, 

 which is here dissected open. {After 

 Fowler.) 



