I5P 



NATURE 



[December 13, 1900 



of a very plastic organisation, that is, one which will 

 respond readily and accurately to the demands of the 

 external conditions of life. For plasticity is clearly only 

 of use (and therefore will come to a like extent within 

 the purview of natural selection) in so far as it will 

 provide the organism with the power of striking the right 

 note in response to a particular call. 



On the other hand, there are plants which may have 

 become, for example, specially selected on account of 

 their ability to flourish in dry, hot, desert lands. Such 

 plants might be expected to retain slender powers of 

 responding in a manner favourable to the continuance of 

 life under opposite hygrometric conditions ; and everyone 

 is aware how extremely intolerant of moisture are the 

 cacti and some other xerophytic plants. Nor is this 

 surprising, seeing how trivial a part the development of 

 a purposeful adaptation to satisfy the needs of a damp 

 environment can have played in their ancestral experi- 

 ence, and indeed the chance of any individual amongst 

 them possessing the power of responding quickly and 

 appropriately to such conditions, to which they are never 

 exposed, is an exceedingly remote one. It is otherwise 

 with species that inhabit regions which, though usually 

 dry, are occasionally or periodically exposed to different 

 conditions. That the plants living in such places, though 

 they may be mainly of a xerophytic habit, nevertheless 

 retain the faculty of withstanding wet is precisely what 

 one would have been led to anticipate, and there are 

 plenty of examples in which the alternation of dry and 

 wet seasons is accompanied by a change in habit analogous 

 to that exhibited by our own trees in summer and winter 

 respectively. An immense weight of evidence has been 

 accumulated by those who have helped to elucidate these 

 matters which goes to prove that the power to vary in 

 any given direction is possessed in an unequal degree, 

 not merely by individuals of the same species, but even 

 in those growing side by side, and thus apparently 

 exposed to very similar conditions. 



This fact at once emphasises the importance of the 

 preexisting internal factor of variation, and it also ex- 

 plains the existence of a criterion which can determine 

 what individuals shall survive in the struggle imposed 

 by new or changing conditions. The particular variation 

 elicited in any given instance is merely the outward and 

 visible sign of the operation of an inward organisation or 

 mechanism. It is the latter which, forming an integral 

 part of the parental constitution, will be transmitted to 

 the offspring. And if those mdividuals which possess the 

 special organisation in the highest degree are thereby 

 enabled to leave the most vigorous or favourably 

 situated descendants, that character, v/hich is its outward 

 token, will become correspondingly strengthened till it 

 comes to form a mark of the race. It is the function of 

 the environment to prove the individual capacity in that 

 contest where the race is emphatically to the swift and 

 the battle to the strong. 



Hence, it would seem that not variability only, but that 

 special (purposeful) form of it which enables so many 

 organisms to make suitable responses to divergent con- 

 ditions of life ought to be, as the outcome of the effect of 

 natural selection, a feature of very general occurrence ; 

 and it ought to be most strongly developed in organisms 

 living under changing or changeable conditions, and such 

 NO. 1624, VOL. 63] 



is found to be the case. One may almost assert that the 

 purposefulness of a particular reaction is a measure of 

 the perfectedness of the stimulable mechanism, itself a 

 heritage transmitted through a long ancestral line of in- 

 dividual bodies. 



This view of the matter is obviously in no wise altered, 

 if we admit the occurrence of sudden or discontinuous 

 variations. For these also are themselves congenital in 

 their origin, and all that the environment can do is to 

 encourage the manifestation of a variation (if a favourable 

 one) in as high a degree as the organism can develop it. 

 Nor is the position affected if we allow that a structural 

 reaction may proximately result from a change in the 

 metabolic processes of the organism, such as Sachs, and 

 Goebel following him, have supposed. In fact, there are 

 some familiar instances which hardly admit of any other 

 explanation, as, for example, when different kinds of galls 

 are produced on the same individual oak-tree by different 

 insects. Facts such as these merely shift inquiry to 

 another stage, and it is certainly not less difficult, in these 

 and similar cases, to account for the particular antecedent 

 reactions going on within the plant in such a way as to 

 produce a substance capable of acting as an appropriate 

 stimulus, which shall provoke a reaction in the plant use- 

 ful to the grub which originates it. 



Into the questions as to the origin of the causes of 

 variations themselves, this is naturally not the place to 

 enter ; nor does a consideration of the problems con- 

 cerned with the nature of those variations which may 

 arise correlatively, or which are more or less obscurely 

 conditioned by remote causes residing within the organism 

 itself, and which may appear suddenly, without any im- 

 mediate reference to their adaptedness to contemporary 

 needs, fall within the scope of this article. It is enough 

 to emphasise the point that the occurrence of purposeful 

 reactions to specific stimuli is really in complete harmony 

 with the operation of natural selection acting through the 

 medium of a congenitally varying organisation. 



Turning again to the subject-matter of Prof. Goebel's 

 book, one finds that not only is it replete with interesting 

 results of biological inquiries, but that there are scattered 

 through its pages excellent little essays on morphological 

 subjects. As an example of the former may be cited 

 the explanation of the mechanism which brings about 

 the dehiscence of the antheridia, a topic which has 

 already formed the basis for investigations published 

 from the author's laboratory. The instance of Azolla, 

 too, in which the lower leaf lobe is shown to have an 

 absorbent function, is attractive when the analogous case 

 of Salvinia is recalled. 



The critical treatment of the structure of the grass 

 embryo is an admirable piece of comparative morphology. 

 Prof. Goebel regards the cotyledon as consisting of the 

 scutellum, epiblast and the sheath or coleoptile. His 

 views are supported by evidence drawn from a consider- 

 ation of a large number of other monocotyledons, 

 especially the Cyperaceas, and whether one agrees with 

 his conclusions or not, one cannot but praise the way in 

 which the evidence for them is collected and marshalled. 

 The views put forward as to the homologies existing 

 between the cells formed within the germinating micro- 

 spores of some of the vascular cryptogams will probably 

 provoke criticism, as will also the suggested phylogeny 



