INTRODUCTORY 19 



variety into nature.' But where the motive power lies, which brings 

 about these transformations from the lowliest to ever higher forms 

 of life, was a question which Treviranus apparently did not venture 

 to discuss. To do this, and thus to take the first step towards 

 a causal explanation of the assumed transformations, was left for 

 his successor. 



Jean Baptiste de Lamarck, born in 1744 in a village of Picardy, 

 was first a soldier, then a botanist, and finally a zoologist. He won 

 his scientific spurs first by his Flora of France, and zoology holds 

 him in honour as the founder of the category of ' vertebrates.' Not 

 that he occupied himself in particular detail with these, but he 

 recognized the close alliance of the classes of animals in question 

 — an alliance which was subsequently expressed by Cuvier by the 

 systematic term ' type ' or ' embranchement.' 



In his Philosophie zoologique, published in 1809, Lamarck set 

 forth a theory of evolution whose truth he attempted to . vindicate 

 by showing — as Treviranus had done before him — that the conception 

 of species, on the immutability of which the whole hypothesis of 

 creation had been based, was an artificial one, read into nature by us ; 

 that sharply circumscribed groups do not exist in nature at all ; and 

 that it is often very difficult, and not infrequently quite impossible, 

 to define one species precisely from allied forms, because it is con- 

 nected with these on all sides by transition stages. Groups of forms 

 which thus melted into one another indicated that the doctrine of 

 the fixity of species could not be correct, any more than that of their 

 absolute nature. Species, he maintained, are not immutable, and are 

 not so old as nature; they are fixed only for a certain time. The 

 shortness of our life prevents our directly recognizing this. ' If we 

 lived a much shorter time, say about a second, the hour-hand of the 

 clock would appear to us to stand still, and even the combined 

 observations of thirty generations would afford no decisive evidence 

 as to the hand's movement, and yet it had been moving.' 



The causes on which, according to Lamarck, the transformation 

 of species, their modification into new species, depends, lie in the 

 changes in the conditions of life which must have occurred ceaselessly 

 from the earliest period of the earth's history till our own day, now 

 here, now there, due in part to changes in climate and in food-supply, 

 in part to changes in the earth's crust by the rising or sinking of 

 land-masses, and so forth. These external changes have sometimes 

 been the direct cause of changes in bodily structure, as in the 

 case of heat or cold; but they have sometimes and much more 

 effectively operated indirectly. Thus changed conditions may have 



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