476 



NATURE 



[Sept. 



vD> 



1888 



This leads me to touch on the great theory which we owe to 

 Mr. Darwin. That theory, I need hardly say, was not merely 

 a theory of descent. This had suggested itself to naturalists in 

 the way I have indicated long before. What Mr. Darwin did 

 was to show how by perfectly natural causes the separation of 

 living organisms into races which at once resemble and yet differ 

 from one another so profoundly came about. Heredity explains 

 the resemblance ; Mr. Darwin's great discovery was that varia- 

 tion worked upon by natural selection explained the difference. 

 That explanation seems to me to gather strength every day, and 

 to continually reveal itself as a more and more efficient solvent 

 of the problems which present themselves to the student of 

 natural history. At the same time, I am far from claiming for it 

 the authority of a scientific creed or even the degree of certainty 

 which is possessed by some of the laws of astronomy. I only 

 affirm that as a theory it has proved itself a potent and invalu- 

 able instrument of research. It is an immensely valuable induc- 

 tion ; but it has not yet reached such a position of certitude as has 

 been attained by the law of gravitation ; and I have myself, in 

 the field of botany, felt bound to protest against conclusions 

 being drawn deductively from it without being subjected to the 

 test of experimental verification. This attitude of mine, which 

 I believe I share with most naturalists, must not, however, be 

 mistaken for one of doubt. Of doubt as to the validity of Mr. 

 Darwin's views I have none : I shall continue to have none till 

 I come across facts which suggest doubt. But that is a different 

 position from one of absolute certitude. 



It is therefore without any dissatisfaction that I observe that 

 many competent persons have, while accepting Mr. Darwin's 

 theory, set themselves to criticize various parts of it. But I 

 must confess that I am disposed to share the opinion expressed 

 by Mr. Huxley, that these criticisms really rest on a want of a 

 thorough comprehension. 



Mr. Romanes has put forward a view which deserves the 

 attention due to the speculations of a man of singular subtlety 

 and dialectic skill. He has startled us with the paradox that 

 Mr. Darwin did not, after all, put forth, as I conceive it was his 

 own impression he did, a theory of the origin of species, but 

 only of adaptations. And inasmuch as Mr. Romanes is of 

 opinion that specific differences are not adaptive, while those 

 of genera are, it follows that Mr. Darwin only really accounted 

 for the origin of the latter, while for an explanation of the 

 former we must look to Mr. Romanes himself. For my part, 

 however, I am altogether unable to accept the premises, and 

 therefore fail to reach the conclusion. Specific differences, 

 as we find them in plants, are for the most part indubitably 

 adaptive, while the distinctive characters of genera and of higher 

 groups are rarely so. Let anyone take the numerous species of 

 some well-characterized English genus — for example, Ramincti- 

 lus ; he will find that one species is distinguished by having 

 creeping stems, one by a tuberous root, one by floating leaves, 

 another by drawn-out submerged ones, and so on. But each 

 possesses those common characters which enables the botanist 

 almost at a glance, notwi'hstanding the adaptive disguise, to 

 refer them to the common genus Raminculus. It seems to me 

 quite easy to see, in fact, .why specific characters should be 

 usually adaptive, and generic not so. Species of any large 

 genus must, from the nature of things, find themselves exposed 

 to anything rather than uniform conditions. They must acquire, 

 therefore, as the very condition of their existence, those adaptive 

 characters which the necessities of their life demand. But this 

 rarely affects those marks of affinity which still indicate their 

 original common origin. No doubt these were themselves once 

 adaptive, but they have long been overlaid by newer and more 

 urgent modifications. Still, Nature is ever conservative, and 

 these reminiscences of a bygone history persist ; significant 

 to the systematic botanist as telling an unmistakable family 

 story, but far removed from the stress of a struggle in which 

 they no longer are called upon to bear their part. 



Another episode in the Darwinian theory is, however, likely 

 to occupy our attention for some time to come. The biological 

 world now looks to Prof. Weismann as occupying the most 

 prominent position in the field of speculation. His theory of 

 the continuity of the germ-plasm has been put before English 

 readers with extreme lucidity by Prof. Moseley. That theory, I 

 am free to confess, I do not find it easy to grasp clearly in all 

 its concrete details. At any rate, my own studies do not furnish 

 me with sufficient data for criticizing them in any adequate way. 

 It is, however, bound up with another theory — the non-inherit- 

 ance of acquired characters— which is more open to general 



discussion. If with Weismann we accept this principle, it 

 cannot be doubted that the burden thrown on natural selection 

 is enormously increased. But I do not see that the theory of 

 natural selection itself is in any way impaired in consequence. 



The question, however, is, Are we to accept the principle ? 

 It appears to me that it is entirely a matter of evidence. It is 

 proverbially difficult to prove a negative. In the analogous case 

 of the inheritance of accidental mutilations, Mr. Darwin con- 

 tents himself with observing that we should be "cautious in 

 denying it." Still, I believe that, though a great deal of pains 

 has been devoted to the matter, there is no case in which it has 

 been satisfactorily proved that a character acquired by an organ- 

 ism has been transmitted to its descendants ; and there is, of 

 course, an enormous bulk of evidence the other way. 



The consideration of this point has given rise to what has 

 been called the new Lamarckism. Now, Lamarck accounted 

 for the evolution of organic Nature by two principles — the tend- 

 ency to progressive advancement and the force of external cir- 

 cumstances. The first of these principles appears to me, like 

 Nageli's internal modifying force, to be simply substituting a 

 a name for a thing. Lamarck, like many other people before 

 him, thought that the higher organisms were derived from others 

 lower in the scale, and he explained this by saying that they had 

 a tendency to be so derived. This appears to me much as if we 

 explained the movement of a train from London to Bath by 

 attributing it to a tendency to locomotion. Mr. Darwin lifted 

 the whole matter out of the field of mere transcendental specu- 

 lation by the theory of natural selection, a perfectly intelligible 

 mechanism by which the result might be brought about. Science 

 will always prefer a material moans operandi to anything so vague 

 as the action of a tendency. 



Lamarck's second principle deserves much more serious con- 

 sideration. To be perfectly fair, we must strip it of the crude 

 illustrations with which he hampered 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 per- 

 petual attempt to reach the foliage of trees, seems almost repug- 

 nant 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 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 satisfac- 

 tory evidence of effects produced in this way. Hoffmann ex- 

 perimented with much pains on plants, and the 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 every-day observation, 

 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 supe- 

 riority of the Darwinian over the Lamarckian theory is, as a 

 working hypothesis, immeasurable. 



The new Lamarckian school, if I understand their views cor- 

 rectly, 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 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 

 Weismann 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 flow from it when 

 subordinated to natural selection. 



