24 THE EVOLUTION OF NATURAL SPECIES. 
followed by the descendants of the same species, he considers it nec- 
essary that they should be brought under the influence of different 
conditions. In other words, divergence of character presupposes ex- 
posure to diverse external conditions; and to account for the various 
forms presented by the different species of the same genus, he assumes 
that they must have been subjected to different forms of natural 
selection. This theory of evolution is in strong contrast with the 
theory which attributes the main factors to powers residing in the 
organism. 
2. Divergence through Variation Protected by Isolation, and Unity 
through Commumity of Descent. 
(1) When water flows down the side of a hill from any given point, 
there is usually but one path that it can take; but when an ani- 
mal with the power of locomotion makes the same descent there may 
be many paths equally open to him, and different individuals of the 
same species, if making the descent, will often take different paths. 
The course of the water is determined by laws that allow of no devia- 
tion; the course of the animal is determined by laws that allow of 
considerable diversity in their results; nevertheless, it is in a measure 
determined, and the laws that determine it are found in the relation 
of the powers of the animal to the natural conditions of the different 
paths. Paths leading down a precipitous descent may be the short- 
est and best for the snail, but impassable for the horse; still both the 
horse and the snail may find many paths that are available, and per- 
haps several that are equally available. The rash horses that attempt 
the precipitous paths break their necks, while the cautious horses 
that take the many safe paths are unharmed. But, shall we therefore 
say that the path taken by each horse making a safe descent is 
wholly determined by external conditions? If precipices prevent 
descent by every path but one, there is no opportunity for varying 
the course, and we regard it as being determined by the external con- 
ditions; but when the way is open in many directions, we would say 
that the one horse being thirsty took the path that leads to the 
spring, another being hungry chose the shortest road to the clover 
field, thus attributing the course of the animal to conditions found in 
the animal. 
This well illtistrates what I believe is true concerning the diver- 
gence of character that results in different specific forms. J belveve 
that the quality, the diversity, and the-rapidity of the varvation depend 
chiefly upon the nature of the organism; and that while the nature of 
the external conditions has power to winnow out whatever forms are 
