CHAPTER UIE: 
THE EVOLUTION OF NATURAL SPECIES. 
I. UNItTy 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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