42 MANUAL OF ZOOLOGY. 



8. By a repetition of this process, "varieties" are first 

 established ; these become permanent, and " races " are pro- 

 duced ; finally, in the lapse of time, the differences thus caused 

 become sufficiently marked to constitute distinct " species." 



9. If we grant that past time has been practically infinite, 

 it is conceivable that all the different animals and plants 

 which we see at present upon the globe, may have been 

 produced by the action of Natural Selection upon the off- 

 spring of a few primordial forms, or, it may be, of a single 

 primitive being. 



Originally, Mr Darwin appears to have believed that 

 " Natural Selection" would alone be found to be a sufficient 

 cause to have given rise to all existing species by a process of 

 Evolution from pre-existing forms. In view, however, of 

 certain objections which had been brought forward, Mr 

 Darwin seems to have abandoned this position ; and a cause 

 supplementary to " Natural Selection " was sought for in what 

 Mr Darwin terms " Sexual Selection." The action of Sexual 

 Selection in a supposed process of Evolution, according to 

 Mr Darwin's views, may be stated in the following two pro- 

 positions : 



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 advan- 

 tages over the others, either in point of strength or activity, 

 or in consequence of the possession of more efficient offensive 

 weapons. There will therefore always be a probability 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 off- 

 spring from the strongest males. The peculiarities which 

 enable certain males to succeed in these contests will, cceteris 

 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, however, 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 among certain species of animals, the 

 females exercise a free choice as to the particular male with 

 which they will pair \ the males being passive agents in the 



