CHAPTER III. 



THE EVOLUTION OF NATURAL SPECIES. 

 I. UNITY AND DIVERSITY. 



The great problems of biology are found in the unity and diversity 

 of organisms. What is the nature and origin of the unity ? What 

 the nature and origin of the diversity? And what the relation of each 

 of these classes of facts to the other? 



1. Darwin's Explanation of the Unity of Organic Forms has been widely 

 adopted by naturalists as by far the most probable theory; but his 

 theory of the causes of the diversity of these forms has not met with the 

 same general acceptance. 



He teaches that the variation, on which natural selection acts, is, 

 for the most part, minute and indefinite variation in any and every 

 direction, and that the progressive accumulation of one series of varia- 

 tions, all tending to the production of a new species, is due to nat- 

 ural selection. If all the offspring of any species were allowed to 

 live out the full measure of their days and should have an equal 

 chance to produce descendants, there would be, according to his 

 theory, no tendency to a change of form; for variations of every 

 kind, having an equal chance, would neutralize the divergent tenden- 

 cies of each other in the general result. 



Fluctuating variability, producing individual variations, is attrib- 

 uted for the most part to the indefinite and indirect influence of 

 changed conditions upon the organism, the forms of variation being 

 chiefly determined by the nature of the organism ; but the trans- 

 formation of a group of associated individuals is attributed to 

 natural selection, which is the effect of external conditions tending 

 to give advantage to the form of individual variation that is best 

 adapted to these conditions. He says, "Chance variation [that is, 

 variation unaided by natural selection] would never account for so 

 habitual and large degree of difference as that between the species of 

 the same genus." Not only the existence of the various species of 

 each genus, but the precise form of each species, and the instincts 

 guiding each, are, therefore, attributed to the determining power of 

 conditions outside of the organism, allowing of but one line of trans- 

 formation in the descendants of any one species exposed to the same 

 conditions. In order that any other line of transformation should be 



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