660 ZOOLOGY SECT. 



changing the colour of their surroundings. A third set of varia- 

 tions probably also occur, though direct evidence is wanting 

 namely, variations which may arise within the sexual cells before 

 the union of ovum and sperm, or which may result from that 

 union. The former two sets of variations are generally spoken of 

 as " acquired characters " new characters acquired during the 

 lifetime of the individual but their nature would be more clearly 

 indicated by terming them extrinsic variations, as contrasted with 

 the intrinsic variations forming the last group. 



The extrinsic variations being brought about by the action of 

 external conditions, their causes are very various. In every such 

 case the organism responds to some persistent external influence 

 by undergoing some more or less persistent change. Mutilations, 

 the rapid mechanical removal or destruction of parts, are here, 

 by the terms of the above definition, excluded from the class 

 of variations altogether, since, though the change involved is 

 frequently permanent, it is effected by an influence which is 

 temporary in its character. This, as will be seen, is of importance 

 in connection with the next question we have to deal with the 

 inheritance of acquired characters. 



Can acquired characters or extrinsic variations be transmitted 



by inheritance ? That they can be is of the essence of Lamarck's 



doctrine of development, which, in fact, may be described as, a 



theory of development by means of the inheritance of extrinsic 



variations, or, as it is sometimes called, use-inheritance. But 



the maintenance of the view that extrinsic variations may 



be transmitted is not inconsistent with the acceptance of 



natural selection as a true cause of evolution. Evolution 



might be supposed to be due to the selection and inheritance 



of both intrinsic and extrinsic variations. From the nature of the 



case, evidence in favour of the inheritance of extrinsic variations 



on the one hand, and the occurrence of intrinsic on the other, 



is extremely difficult to obtain. One or the other must occur, or 



there would be no evolution. But to prove in any given case that 



a change is due to the one factor rather than to the other, is 



extremely difficult. When a character not present in the parents 



appears in the offspring, there is, to begin with, great difficulty in 



proving that it is really new : characters not present in the 



parents are known to be frequently inherited from a more or less 



remote ancestor. But, if we suppose it to be established that the 



character is a new one (and absolutely new characters must 



appear, or we should have no evolution), then it would require a 



very accurate knowledge of all the circumstances to enable us to 



be certain whether the appearance of the character is not due 



to the action of some external influence on the parent, either 



during development or in the adult state, rather than to a change 



arising within the reproductive cells. Instances are frequently 



