622 ZOOLOGY SECT. 



or less marked extent by placing it under different conditions. 

 Another set of variations produced by the action of external 

 influences on the organism only appears if the action takes place 

 in the course of development at one stage or another between the 

 oosperm and the adult. Of the occurrence of both these forms of 

 variation we have direct and positive evidence. It is a familiar 

 fact that increased exercise of a part tends to an increase in the 

 bulk of its muscles. The colours and markings of certain Fishes 

 can be altered at will (of coarse within certain limits) by changing 

 the material on the bottom of the aquarium in which they are 

 confined ; the colours of many Caterpillars may be altered by 

 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 



