BOTANICAL BIOLOGY. 409 



sistently stretching the skin between its toes, or that the neck of a 

 giraffe was elongated in the perpetnal attempt to reach the foliage of 

 trees, seems almost repugnant to common sense. But the idea that 

 changes in climate and food — i. e. in the conditions of nutrition gen- 

 erally — may have some slow but direct iutluence on the organism seems, 

 on a superficial view, so plausible, that the mind is very prone to accept 

 it. Mr. Darwin has himself frankly admitted that he thought he had 

 not attached sufficient weight to the direct action of the environment. 

 Yet it is extremely difficult to obtain satisfactory evidence of effects 

 liroduced in this way. Hoffmann experimented with much pains on 

 plants, and the results were negative. And Mr. Darwin confessed that 

 Hoffmann's paper had "staggered" him. 



Organic evolution still seems to me therefore to be explained in the 

 simplest way as the result of variation controlled by natural selection. 

 Now, both these factors are perfectly intelligible things. Variation is 

 a mere matter of every-day observation, and the struggle for existence, 

 which is the cause of which natural selection is the etfect, is equally so. 

 If we state in a parallel form the Lamarckian theory, it amounts to a 

 tendency controlled by external forces. It appears to me that there is 

 no satisfactory basis of fiict for either factor. The practical superiority 

 of the Darwinian over the Lamarckian theory is (as a working hypothe- 

 sis) immeasurable. 



The new Lamarckian school, if I understand their views correctly, 

 seek to re-introduce Lamarck's " tendency." The fact has been admitted 

 by Mr. Darwin himself that variation is not illimitable. No one, in 

 fact, has ever contended that any type can be reached from any point. 

 For example, as Weismann puts it, " Under the most favorable circum- 

 stances, a bird can never become transformed into a mammal." It is 

 deduced from this that variation takes place in a fixed direction only, 

 and this is assumed to be due to an innate law of development, or as 

 Weismann has termed it, a " phyletic vital force." But the introduction 

 of any such directive agency is suj)erfluous, because the limitation of 

 variability is a necessary consequence of the physical constitution of 

 the varying organism. 



It is sui)posed however by many people that a necessary part of 

 Mr. Darwin's theory is the explanation of the phenomenon of variation 

 itself. But really this is not more reasonable than to demand that it 

 should explain gravitation or the source of solar energy. The investi- 

 gation of any one of these phenomena is a matter of first-rate importance. 

 But the cause of variation is perfectly independent of the results that 

 flow from it when subordinated to natural selection. 



Though it is difl3cult to establish the fact that external causes pro- 

 mote variation directly, it is worth considering whether they may not 

 do so indirectly. Weismann, like Lamarck before hini, has pointed out, 

 as others have also done, the remarkable persistence of tne plants and 

 animals of Egypt; and the evidence of this is now even stronger. We, 



