432 HOMEK WHEELON 



duced by the* two typos of germinal tissue; consequently, the somatic 

 changes necessary to insure fertilization were brought about. The appear- 

 ance of two types of reproductive tissue associated with somatic divergences 

 established sex. Therefore, sex may be defined as dependent upon the 

 sum total of the somatic characteristics and differences associated with 

 the reproductive tissue. Those somatic characteristics accompanying re- 

 productive tissue which gives rise to ova, define the female sex; those 

 accompanying reproductive tissue which give rise to spermatozoa, define 

 the male sex. However, as will appear later, sex characters are not de- 

 pendent, in all forms of life, upon the presence of active germ tissue, nor 

 the type of reproductive tissue present. 



Evolution of Genital Organs. As animals assumed greater propor- 

 tions the matter of liberating germ cells became of primary importance, 

 hence, there arose systems of outlets genital organs. In lowly forms 

 of life the germ cells are simply extruded from the body surface. Fertil- 

 ization, in such forms, is brought about by the fortuitous meeting of egg 

 and sperm. In higher forms copulatory organs have been evolved, by 

 means of which the spermatozoa are transmitted by the male directly to 

 the female. Such adaptations insure the more certain fertilization of 

 the egg. 



Secondary Sexual Characteristics. In addition to the evolution of 

 male and female genital organs, arose other phenomena by which the 

 sexes are characterized. Such characters were designated by J. Hunter as 

 Secondary Sexual Characteristics. This term embraces all those specific 

 differences between the male and female of the species which are not 

 directly concerned with the processes of reproduction. Such characters 

 are usually more elaborate in the male than in the female. Familiar 

 examples of these characters are found in insects and vertebrates, but 

 are rare or absent in the lower invertebrates. The horns of the stag, the 

 mane of the lion, the great variation in color among birds, the phos- 

 phorescent organs of the firefly, and the distribution of hair in man, are 

 typical examples of secondary sexual characteristics. 



Sexual Instincts. Their Occurrence and Purpose. In addition to the 

 structural and functional differences between the sexes have arisen the 

 sexual instincts. In the lower forms these are absent and the meeting 

 of egg and sperm remains a matter of chance. In the higher forms these 

 instincts, or, as I prefer to call them, "reaction impulses/' bring the male 

 and female together at the breeding seasons; control the behavior of the 

 individuals towards each other courtship, the union of the sexes cop- 

 ulation or its equivalent ; and, finally, direct the various activities in- 

 volved in the building of the nest and the rearing of the young. 



Sex Characteristics and the Breeding Season. In many types of 

 animals the sexual instincts and certain somatic characters are accentuated 

 or appear only during the breeding season. Many of the secondary sexual 



