TRANSACTIONS OF SECTION D. 693 



it. To suggest that a bird became web-footed by persistently stretching the skin 

 between its toes, or that the neck of a giraffe was elongated in the perpetual 

 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 

 generally — may have some slow but direct influence on the organism 8eem.s, 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 difiicult to 

 obtain satisfactory evidence of eflects produced in this way. Hoft'mann experimented 

 -with much pains on plants, and tlie results were negative. And Mr. Darwin 

 confessed that Hoffmann's paper had ' staggered' him. 



Organic evolution still, therefore, seems to me 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 everyday observa- 

 tion, and the struggle for existence which is the cause, of which natural selection 

 is the effect, 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 fact for either factor. The practical superiority of the 

 Darwinian over the Lamarckian theory is, as a working hypothesis, 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 him- 

 self that variation is not illimitable. No one, in fact, has ever contended that any 

 type can be reached from any point. For example, as "Weissmann puts it, ' under 

 the most favourable circumstances 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 

 Weissmann has termed it, a ' phyletic vital force.' But the introduction of any 

 ■such directive agency is superfluous, because the limitation of variability is a 

 necessary consequence of the physical constitution of the varying organism. 



It is supposed, 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 investigation 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 tlow from it when subordinated to natural selection. 



Though it is diflicult to establish the fact that external causes promote varia- 

 tion directly, it is worth considering whether they may not do so indirectly. 

 AVeissmann, like Lamarck before him, has pointed out, as others have also done, the 

 remarkable persistence of the plants and animals of Egypt; and the evidence of 

 this is now even stronger. We owe, at Kew, to the kindness of Dr. Schweinfurth, 

 a collection of specimens of plants from Egyptian tomte which are said to be as 

 much as 4,000 years old. They are still perfectly identifiable, and, as one of my 

 predecessors in this chair has pointed out, thej- difl'er in no respect from their 

 living representatives in Egypt at this day. The explanation which Lamarck gave 

 of this fact 'may well,' says Sir Charles Lyell, 'lay claim to our admiration.' He 

 attributed it, in effect, to the persistence of the physical geography, temperature, 

 and other natural conditions. The explanation seems to me adequate. The 

 ■plants and animals, we may fairly assume, were, 4,000 years ago, as accurately 

 adjusted to the conditions in which they then existed as the fact of their persistence 

 in the country shows that they must be now. Any deviation from the type that 

 existed then would either, therefore, be disadvantageous or indifierent. In the 

 former case it would be speedily eliminated, in the latter it would be swamped by 

 cross-breeding. But we know that if seeds of these plants were introduced into 

 our gardens we should soon detect varieties amongst their progeny. Long obser- 

 vation upon plants imder cultivation has always disposed me to think that a 

 change of external conditions actually stimulated variation, and so gave natural 

 selection wider play and a better chance of re-establishing the adaptation of the 

 organism to them. Weissmann explains the remarkable fact that organisms may 

 for thousands of years reproduce themselves unchanged by the principle of the 



