330 



NATURE 



[Fed. 15, 1877 



ing much for different organisms — in the sexual elements 

 which take part in it is necessary. And in so far as Mr. 

 Spencer's theory suggests an analogy to chemical change, 

 it is perhaps leading us away from the direction of real 

 explanation altogether. 



The use of the phrase "mean differentiation" perhaps 

 conveniently expresses Mr. Darwin's ingenious and most 

 probable correlation of the facts of hybridisation with 

 those of self-fertilisation. 



" It is an extraordinary fact that with many species, 

 flowers fertilised with their own pollen are either absolutely, 

 or in some degree, sterile ; if fertilised with pollen from 

 another flower on the same plant they are sometimes, 

 though rarely, a little more fertile ; if fertilised with 

 pollen from another individual or variety of the same 

 species, they are fully fertile ; but if with pollen from a 

 distinct species, they are sterile in all possible degrees 

 until utter sterility is reached. We thus have a long 

 series with absolute sterility at the two ends ; at one end 

 due to the sexual elements not having been sufficiently 

 differentiated, and at the other end to their having been 

 differentiated in too great a degree, or in some peculiar 

 manner " (pp. 455, 456). 



In this mode of regarding phenomena which at first 

 hardly seem to have anything in common, and embracing 

 them under a single " expression," there is a neatness 

 quite mathematical. Mr. Darwin admits, however, with 

 characteristic frankness that in thus breaking down the 

 fundamental difference between species and varieties, he 

 traverses a prejudice which " it will take many years to 

 remove " (p. 467). 



But it is possible to go even further and regard ga- 

 mogenesis and agamogenesis themselves as particular 

 cases of a generalised process. Every organism, whether 

 sexually produced or not, may be regarded as an aggre- 

 gate of cells derived from a single mass of protoplasm 

 which has undergone repeated division. Fertilisation, as 

 Prof. Huxley has remarked,' is only " one of the many 

 conditions which may determine or affect that process.'' 

 And this remark probably supplies the explanation of the 

 undoubted fact that amongst flowering plants as in every 

 other part of the vegetable kingdom, there is every grada- 

 tion between plants which are simply incapable of self- 

 fertilisation and therefore would die out if they were not 

 perpetually crossed, and others in which self-fertilisation 

 is the rule. 



" Some few plants, for instance, Ophrys apifera^ have 

 almost certainly been propagated in a state of nature for 

 thousands of generations without having once been inter- 

 crossed ; and whether they would profit by a cross with a 

 fresh stock is not known. But such cases ought not to 

 make us doubt that, as a general rule, crossing is bene- 

 ficial, any more than the existence of plants which in a 

 state of nature are propagated exclusively by rhizomes, 

 stolons, &c. (their flowers never producing seeds), should 

 make us doubt that seminal generation must have some 

 great advantage, as it is the common plan followed by 

 nature" (p. 439). 



Still there is room for believing that nature may be able 

 to give more or less freely to plants, but in some other 

 way, those benefits which gamogenesis, especially in its 

 more differentiated forms, undoubtedly confers. It may 

 be one of nature's favourite expedients, and yet not the 

 only one. It is highly important to bear this in mind 

 and to keep clearly in view what it is exactly that Mr. 

 Darwin has done. He has explored, and in a manner 



} " Encylcopaedia Britannica," Art. Biology, p, 687. 



which had never been attempted, much less accomplished 

 before, the precise utility of cross-fertilisation, and has 

 consequently given enormously increased force to all argu- 

 ments drawn from the adaptive arrangements that promote 

 it by demonstrating their extreme urgency. But he has not 

 tied nature's hands to doing her work with this implement 

 alone, and therefore he is not open to the objection which 

 some persons will probably urge, that cross-fertilisation 

 cannot be so important, seeing that many plants get 

 on apparently very well without it. This is, indeed, as 

 if one were to argue that the printing-press cannot have 

 had the influence attributed to it, seeing that there have 

 been those v/ho expressed their meaning excellently well 

 with the help of the fore-finger and some sandy soil. 



The evidence which Mr. Darwin has collected leads 

 almost irresistibly to the conclusion that the benefit de- 

 rived from gamogenesis does not depend upon any 

 mysterious property inherent in the process itself, but 

 that '• change " is to be regarded as at the bottom of 

 the benefit derived from it ; intercrossing, in fact, ceases 

 to be beneficial if the plants crossed have been for many 

 generations exposed to the same conditions. The advantage 

 is, in fact, of the same kind as that which all organisms 

 seem to derive from " an occasional and slight change in 

 the conditions of life." " But the offspring from a cross 

 between organisms which have been exposed to different 

 conditions [and therefore differentiated] profit in an in- 

 comparably higher degree than do young or old beings 

 from a mere change in their conditions" (pp. 454, 455), 

 and the reason is that "the blending together of the 

 sexual elements of two differentiated beings will affect the 

 whole constitution at a very early period of life, whilst 

 the organisation is highly flexible." But as change may 

 be of the most variable amount, the corresponding diffe- 

 rentiation maybe equally variable. In some cases it must 

 be exceedingly small ; amongst the Conjui^atcc, for exam- 

 ple, in Rhynchonc})ia, two adjacent cells of a filament unite 

 by small lateral processes which bridge over the interven- 

 ing septum. And the bridge being very narrow, one 

 cell is forced to become the recipient of the contents of 

 the other and the sexual differentiation of the two conju- 

 gating cells is thereby established. In Vaucheria, where 

 the protoplasm is continuous through the whole vegetative 

 portion of the filamentous organism, the sexual organs 

 are formed by small adjacent processes which are merely 

 parted off from the common protoplasm of the filament 

 which bears them. This must also be an extremely close 

 case of self-fertiHsation, but as fertilisation is effected 

 by motile antherozoids, there is a remote possibility of an 

 occasional cross. The hermaphrodite condition in such 

 cases may easily be conceived to have been developed 

 from a stage in which conjugation alone obtains. ^ 



It would not be difficult to show that all through the vege-Jp 

 table kingdom the hermaphrodite condition precedes the ' 

 dioecious. Thus in ferns where the sexual organs are deve- 

 lopments of epidermal processes on the peculiar inter- 

 mediate generation known as the prothallium, there is 

 almost every condition which is met with in flowering 

 plants. The female organs (archegonia), however, require 

 more than one layer of cells for their ultimate develop- 

 ment, and are consequently matured later than the male 

 organs (antheridia). Hence ferns tend to be proteran- 

 drous and therefore functionally dioecious; and as it fre- 



