224 LAMARCK AND DARWIN 



employed by Nature to diversify species, a means which 

 comes into play whenever the environment changes. The 

 cause of the great diversity shown by animal species is indeed 

 ultimately to be sought in the environment. As the imperfect 

 and earliest forms developed they spread over the earth and 

 invaded the utmost corners of it : " One can imagine what 

 an enormous variety of habitats, stations, climates, available 

 foods, environing media, etc., animals and plants have had to 

 endure, as the existing species were forced to change their 

 place of abode. And although these changes have taken 

 place with extreme slowness . . . their reality, necessitated 

 by various causes, has none the less induced the species 

 affected by them slowly to change their manner of life and 

 their habitual actions. Through the effects of the second and 

 third of the laws cited above, these induced activity-changes 

 must have brought into being new organs, and must have 

 been able to develop them further if more frequent use was 

 made of them ; they must in the same way have been 

 capable of bringing about the degeneration and finally the 

 complete disappearance of existing organs which had become 

 useless " (p. 161). 



On the other hand, if the environment does not change, 

 species remain constant. 



It is to be noted that change in environment is rather the 

 occasion than the cause of modification ; the environment 

 induces the organism to change its habitual way of life ; it 

 sets up new needs, to satisfy which the organism must modify 

 its structure. It is the organism that takes the active part 

 in all this, the action of the environment is indirect. 



Of Lamarck's fourth law, which asserts the transmission 

 of acquired characters, little need here be said in the way of 

 exposition. Upon the truth of it depends of course Lamarck's 

 whole theory. He himself never dreamed that anyone would 

 ever dispute it. 



Lamarck sums up as follows: " By the four laws which I 

 have just enunciated all the facts of organisation seem to me 

 to be easily explained; the progression in the complexity 

 of organisation of animals, and in their faculties, seems 

 to me easy to conceive ; so, too, the means which Nature 

 has employed to diversify animals, and bring them to the 



