194 PRINCIPLES OF ANIMAL BIOLOGY 



lation) or are deposited by the male during or just after amplexus. Id 

 the latter case the female secures them immediately or after an interval. 



Do the Methods of Insuring Fertihzation Exhibit an Evolutionary 

 Series? — 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. Thus, as pointed 

 out above, the last of the four types of habit recognized, which involves 

 more specialized behavior than any of the others, is employed by some 

 parasitic worms, some snails, and the insects, which stand low or inter- 

 mediate in the animal series, and by reptiles, birds and mammals which 

 belong to the highest phylum. Furthermore, most of the fishes and 

 amphibia use the second and third of these methods while some members 

 of each of these groups follow the fourth method. 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. If, there- 

 fore, breeding habits are the result of an evolutionary process that process 

 has small relation to the evolution of structure. 



It is worthy of note, however, that among aquatic or amphibious 

 forms the habit prevails of depositing the sperms and eggs freely in the 

 water or in immediate proximity to each other, or depositing the sperms 

 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 essential to 

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

 aquatic forms at least the fertilized eggs can endure the water for a pro- 

 longed period. 



Place of Development. — From the description of the methods of 

 insuring fertilization it will be seeii that the eggs may be fertilized either 

 before or after they are laid. That is, fertilization is either internal 

 or external. It is now to be pointed out that when fertilization is internal 

 the eggs may be retained for a long time after fertilization, or they may 

 be laid very soon thereafter. Whatever period of time the eggs remain 

 in the organs of the female after fertilization is utilized in development, so 

 that the embryo may be far advanced before it is separated from the 

 mother, or it may have attained only an early stage of development, or 

 development may scarcely have started. Thus, in most of the insects 

 and in all of the birds the eggs are laid soon after fertilization. In these 

 cases only a few divisions of the egg, or of its nucleus, have taken place 

 at the time of oviposition, or it may not have divided even once. On 

 the contrary, development may proceed until a well formed embryo 

 is produced, and then the eggs are laid; this occurs in some of the sala- 



