May, 1896. LYELL AND LAMARCKISM. 327 



2. " When the change of situation which they can endure is great, 

 it is usually attended by some modifications of the form, colour, size, 

 structure, or other particulars ; but the mutations thus superinduced 

 are governed by constant laws, and the capability of so varying forms 

 part of the permanent specific character. 



3. " Some acquired peculiarities of form, structure, and instinct, 

 are transmissible to the offspring ; but these consist of such qualities 

 and attributes only as are intimately related to the natural wants and 

 propensities of the species. 



4. " The entire variation from the original type, which any given 

 kind of change can produce, may usually be effected in a brief period 

 of time, after which no further deviation can be obtained by con- 

 tinuing to alter the circumstances, though ever so gradually ; indefinite 

 divergence, either in the way of improvement or deterioration, being 

 prevented, and the least possible excess beyond the defined limits 

 being fatal to the existence of the individual. 



5. " The intermixture of distinct species is guarded against by the 

 aversion of the individuals composing them to sexual union, or by 

 the sterility of the mule offspring. It does not appear that true 

 hybrid races have ever been perpetuated for several generations, even 

 by the assistance of man ; for the cases usually cited relate to the 

 crossing of mules with individuals of pure species, and not to the 

 intermixture of hybrid with hybrid. 



6. " From the above considerations it appears that species have 

 a real existence in nature ; and that each was endowed, at the time of 

 its creation, with the attributes and organisation by which it is now 

 distinguished." 



I think there can be no dispute, with this summary before us, 

 that Lyell's objection to Lamarck's views was, not that the effects of 

 the environment were non-adaptive, but that they were permanently 

 limited in extent. The doctrine of the fixity and essential immuta- 

 bility of species having been abandoned, as it ultimately was by Lyell, 

 it follows that, in the judgment of both Lyell and Darwin, there was 

 no ultimate limit to the extent of adaptive and hereditary modification 

 that could be produced directly by changes in the environment. 



Professor Brooks's argument having been shown not to have the 

 authority of Lyell, which he claims for them, we will proceed to 

 consider them on their own merits. He takes the case of the 

 strengthening of muscles by exercise, which he admits to be an 

 adaptive, i.e., a. beneficial, change in relation to the conditions calling 

 it forth, " one of the simplest examples of the beneficial effects of 

 the conditions of life." But this, he tells us, is not an example of 

 the direct action of the environment on the organism. On the con- 

 trary, the increase and improvement of the muscle are due to the 

 increased supply of nourishment produced by exercise ; and the fact 

 that exercise causes an increased circulation of blood in the muscle 

 depends upon structural adjustments, which themselves constitute 

 an adaptation, originally, I suppose, due to congenital variation and 

 selection. Now I think it may well be held that the supply of 

 nourishment is not the same thing as assimilation and growth ; and 

 the assimilation and growth G f the muscle under stimulus must be 



