234 THE HISTORY OF CREATION. 
phenomena, which are usually divided into two quite 
distinct groups. Naturalists, as a rule, have distinguished, 
first, those variations of organisms which are produced 
directly by the permanent influence of external conditions 
(by the constant action of nutrition, of climate, of surround- 
ings, ete.), and secondly, those variations which arise from 
habit and practice, from accustoming themselves to definite 
conditions of life, and from the use and non-use of organs. 
The latter influences have been set forth especially by 
Lamarck as important causes of the change of organic 
forms, while the former have for a very long time been 
recognized as such more generally. 
The sharp distinction usually made between these two 
groups of cumulative adaptation, and which even Darwin 
still maintains, disappears as soon as we reflect more 
accurately and deeply upon the real nature and causal 
foundation of these two, apparently very different, series 
of adaptations. We then arrive at the conviction that in 
both cases there are always two different active causes to 
be dealt with: on the one hand the external influence or 
action of adaptative conditions of life, and on the other 
hand the internal reaction of the organism which subjects 
and adapts itself to that condition of life. If cumulative 
adaptation is considered from the first point of view alone, 
and the transforming actions of the permanent external con- 
ditions of life are traced to those conditions solely, then the 
principal stress is laid unduly upon the external factor, and 
the necessary internal reaction of the organism is not taken 
into proper consideration. If, on the other hand, cumulative 
adaptation is unjustly regarded solely in relation to its 
second factor, and the transforming action of the organism 
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