138 ELEMENTS OF BIOLOGY. 



Evolution, according to Mr Darwin's views, may be stated 

 in the following two propositions : — 



a. The males of many species of animals are known to 

 engage in very severe contests for the possession of the 

 females, these latter yielding themselves to the victor. In 

 such contests certain males will inevitably have certain ad- 

 vantages over the others, either in point of strength or ac- 

 tivity, or in consequence of the possession of more efficient 

 offensive weapons. There will therefore always be a pro- 

 bability that certain males will get possession of the females 

 in preference to others ; and thus there will be a tendency 

 in the individuals of many species of animals to secure a 

 preponderance of offspring from the strongest males. The 

 peculiarities which enable certain males to succeed in these 

 contests will, C(zteris paribus, be transmitted to their male 

 offspring, and in this way variations may be perpetuated, 

 initiated, or intensified. 



b. In the preceding cases, the females are believed to be 

 perfectly passive, and the selection is a " natural " one, the 

 final result depending solely upon the natural advantages 

 which certain males possess over others in actual combat. 

 It is alleged, ho\wever, that there are other cases in which 

 the selection is truly " sexual," since its result is determined 

 by spontaneous preference, and not by brute force alone. 

 It is asserted, namely, that amongst certain species of ani- 

 mals, the females exercise a free choice as to the particular 

 male with which they will pair; the males being passive 

 agents in the matter, except in so far as each uses, or may 

 use, his utmost exertions to secure that the choice of the 

 female may fall upon him. The circumstances supposed 

 to influence, and ultimately determine, the choice of the 

 female, are of course, in the main, the pergonal attractions 

 of some particular male, the female being captivated by 

 some " beauty of form, colour, odour, or voice," which such 

 a male may possess. 



If it be admitted that the females of some of the lower 



