[97] THE I VOLUTIOX OF THE FINS OF FISHES. 1077 



naturhlstorischen Art, Miiucben, 1805) who supposes that each orgau- 

 isin has in itself a teudeucy to vary in a definite direction, to increase 

 the morphological difl'erentiatiou, or, as it is commonly expressed, to per- 

 fect itself. The tendency to vary in a definite or in an indefinite direc- 

 tion is, however, a purely transcendental statement of what are seem- 

 ingly facts. Tendency, as we will soon find, is a word which may be 

 made to cover a great deal of ignorance. Tendencies must have causes. 

 If I say that an unsupported stone has a tendency to fall, I do not tell 

 what it is that causes it to fall ; it is certainly not the tendency which 

 makes it do so, but a specific force, which acts in such a way as to pull 

 the earth and the stone toward each other. 



Huxley* states the grounds of the divergence of opinion amongst 

 those who accept the doctrine of evolution in its main. features. Three 

 views may be taken of the causes of variations: 



"a. In virtue of its molecular structure, the organism may tend to 

 vary. This variability may either be indefinite or may be limited to 

 certain directions by intrinsic conditions. In the former case the result 

 of tlie struggle for existence would be the survival of the fittest amongst 

 an indefinite number of varieties; in the latter case it would be the sur- 

 vival of the fittest among a certain set of varieties, the nature and num- 

 ber of which would be predetermined by the molecular structure of the 

 organism. 



"6. The organism mny have no intrinsic tendency to vary, but varia- 

 tion may be brought abont by the influence of conditions external to it. 

 And in this case, also, the variability induced may be either indefinite 

 or defined by intrinsic Ihnitation. 



"c. The two former cases may be combined, and variation may to 

 some extent depend upon intrinsic, and to some extent upon extrinsic, 

 conditions. 



"At present it can hardly be said that such evidenceas would justify 

 the positive adoption of any one of these views exists." 



These statements of the grounds of the hypothesis of evolution by one 

 of the greatest biological thinkers of our time are, like everything else 

 which he writes, logical and to the point; but let us see how Lamarck 

 had already appreciated the interaction of the above-mentioned intrinsic 

 and extrinsic forces in his second law {An. sans. Vert. I, 57) : 



" Second law : The production of a new organ in an animal body re- 

 sults from a new need which has arisen unexpectedly, and which con- 

 tinues to make itself felt, and which causes the new movements to be 

 made to which this need gave origin and maintained." 



Totally new organs, with naw functions, are, as is well known, rarely 

 developed ; but the method of evolution seems to be to seize upon an 

 organ already developed and modify it by adding or subtracting to or 

 from its bulk, or so modifyiug it as to amount to a metamorphosis of 



*Auat. of Iiivertebrated Animals, pp. 41,42. 



