CHAPTER IX 

 THE BREEDING HABITS OF ANIMALS 



Reproduction in which both sexes are involved is dependent upon the 

 uniting of the germ cells, proper conditions for the development of the 

 fertihzed egg, and conditions suitable for the development of the imma- 

 ture animal. The parents do more or less to insure the union of the germ 

 cells, to provide proper conditions for the development of the fertilized 

 egg, to guard against accident to the immature offspring, and to help it 

 over the period of its own helplessness. These services of the parents 

 are habitual, and are known collectively as breeding habits. A consider- 

 able amount of data on breeding habits has been accumulated by zoolo- 

 gists, but this is widely scattered in the hterature and is very fragmentary. 

 The habits of relatively few forms are known in detail. 



Breeding habits in the animal kingdom are exceedingly varied. 

 There are two apparent reasons for this variety. First, different forms 

 have different modes of hfe, and the breeding habits must be suited to the 

 manner of hving if they are to accomphsh their purpose. Second, the 

 increasing complexity attained in the higher forms of life apparently 

 necessitates in them a longer period of prenatal development. At least, 

 the development before birth or hatching is longer in the complex forms 

 than in the simpler ones, and the length of time required for this develop- 

 ment seems to be related to the place of the forms in the animal scale. 

 There may be other reasons than these two; but whatever the explana- 

 tion of them, the breeding habits of animals exhibit very great diversity. 

 The differences in habits are not characters that distinguish large groups, 

 as phyla^and classes, from one another; for within these and smaller 

 groups there is considerable dissimilarity in breeding habits. Even the 

 several species of a single genus may employ very different means of 

 assisting the processes of reproduction and development. Because of 

 this diversity no attempt will be made to describe in detail the various 

 breeding habits of animals. The endeavor will rather be to bring order 

 out of seeming chaos, to classify and summarize, and to introduce just 

 enough detail to illustrate in concrete manner the several types of breed- 

 ing behavior. Habits dealing with the union of the germ cells are de- 

 scribed first, after which those relating to the fertihzed eggs and the 

 young are discussed. 



Methods of Insuring Fertilization. — Since bisexual reproduction is the 

 union of protoplasm from two individuals or the union of differentiated 

 cells from the same parent the first essential is that the uniting elements 



1 For the significance of the terms phylum, class and genus, see pp. 263-264. 



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