4 A POSTULATE. [introd. 



The suggestion which thus forms the common ground of these 

 theories is this : — May not the Specific Differences between Species 

 and Species have come about through and be compounded of the 

 individual differences between parent and offspring ? May not 

 Specific Differentiation have resulted from Individual Variation ? 

 This suggestion has been spoken of as the Doctrine of Common 

 Descent, for it asserts that there is between living things a 

 community of descent. 



In what follows it will be assumed that this Doctrine of 

 Descent is true. It should be admitted from the first that the 

 truth of the doctrine has never been proved. There is never- 

 theless a great balance of evidence in its favour, but it finds its 

 support not so much in direct observation as in the difficulty of 

 forming any alternative hypothesis. The Theory of Decent in- 

 volves and asserts that all living things are genetically connected, 

 and this principle is at least not contrary to observation ; while 

 any alternative hypothesis involves the idea of Separate Creation 

 which by common consent is now recognized as absurd. In favour 

 of the Doctrine of Common Descent there is a balance of evidence: 

 it is besides accepted by most naturalists ; lastly if it is not true 

 we can get no further with the problem : but inasmuch as it is 

 unproven, it is right that we should explicitly recognize that it is 

 in part an assumption, and that we have adopted it as a pos- 

 tulate. 



The Doctrine of Descent being assumed, two chief solutions of 

 the problem have been offered, both starting alike from this 

 common ground. Let us now briefly consider each of them. 



A. Lamarck's Solution. So many ambiguities and pitfalls are 

 in the way of any who may try to re-state, in a few words, the 

 theory propounded in the Philosoplvie Zoologique, that it is with 

 great diffidence that the following account of it is given. 



Lamarck points out that living things can in some measure 

 adapt themselves both structurally and physiologically to new 

 circumstances, and that in certain cases the adaptability is present 

 in a high degree. He suggests that by inheritance and perfection 

 of such adaptations they may have become what they are, and that 

 thus specific forms and mechanisms have been produced, as it were, 

 by sheer force of circumstances. On this view it is assumed that 

 to the demands made on it by the environment the organism 

 makes an appropriate structural and physiological response ; in 

 other words, that there is in living things a certain tension, by 

 which they respond to environmental pressure and fit the place 

 they are in, somewhat as a fluid fits a vessel. 



This is not, I think, a misrepresentation of Lamarck's theory. 

 It amounts, in other words, to a proposal to regard organisms as 

 machines which have the power of Adaptation as one of their 

 fundamental and inherent qualities or attributes. 



