236 



NATURE 



{July 13, 1876 



can look for nothing beyond a simplification of the 

 problem, and a reduction of it to the same category with 

 certain other problems which also admit of hypothetical 

 solution only. If an hypothesis which certain other wide- 

 spread phenomena have already thrust upon us, can be 

 shown to render the phenomena of heiedity more intel- 

 ligible than they at present seem, we shall have reason to 

 entertain it. The applicability of any method of inter- 

 pretation to two different but allied classes of facts is 

 evidence of its truth. The power which organisms dis- 

 play of reproducing lost parts, we saw to be inexplicable 

 except on the assumption that the units of which any 

 organism is built have an innate tendency to arrange 

 themselves into the shape of that organism. We inferred 

 that these units muse be the possessors of special polari- 

 ties, resulting from their i^pecial structures ; and that by 

 the mutual play of their polarities they are compelled to 

 take the form of the species to which they belong. And 

 the instance of the Begonia phylloma7iiaca left us no 

 escape from the admission that the ability thus to 

 arrange themselves is latent in the units in every un- 

 differentiated cell. , . . The assumption to which we 

 seem driven by the ensemble of the evidence, is that 

 sperm-cells and germ-cells are essentially nothing more 

 than vehicles, in which are contained small groups of the 

 physiological units in a fit state for obeying their pro- 

 clivity towards the structural arrangement of the species 

 they belong to. . . . If the likeness of offspring to parents 

 is thus determined, it becomes manifest, a priori, that 

 besides the transmission of generic and specific pecu- 

 liarities, there will be a transmission of those individual 

 peculiarities which, arising without assignable causes, are 

 classed as * spontaneous '. . . . 



"That changes of structure caused by changes of 

 action must also be transmitted, however obscurely, from 

 one generation to another, appears to be a deduction 

 from first principles — or if not a specific deduction, still, 

 a general implication. . . . Bringing the question to its 

 ultimate and simplest form, we may say that as on the 

 one hand physiological units will, because of their special 

 polarities, build themselves into an organism of a special 

 structure, so on the other hand, if the structure of this 

 organism is modified by modified function, it will impress 

 some corresponding modification on the structures and 

 polarities of its units. The units and the aggregate must 

 act and re-act on each other. The forces exercised by 

 each unit on the aggregate, and by the aggregate on each 

 unit, must ever tend towards a balance. If nothing 

 prevents, the units will mould the aggregate into a form 

 in equilibrium with their pre-existing polarities. If 

 contrariwise, the aggregate is made by incident actions 

 to take a new form, its forces must tend to re-mould the 

 units into harmony with this new form ; and to say that 

 the physiological units are in any degree so re-moulded 

 as to bring their polar forces towards equilibrium with 

 the forces of the modified aggregate, is to say that when 

 separated in the shape of reproductive centres, these units 

 will tend to build themi^elves up into an aggregate modi- 

 fied in the same direction." (P. 256.) 



Thus, then, Mr. Herbert Spencer definitely assumes an 

 order of molecules or units of protoplasm — lower in 

 degree than the visible cell-units orplastids — to the "polar 

 forces" of which and their modification by external 

 agencies and interaction, he ascribes the ultimate respon- 

 sibility in reproduction, heredity, and adaptation. 



I am unable to say whether Mr. Darwin was acquainted 

 with or had considered Mr. Herbert Spencer's hypothesis 

 of physiological units, when in 1868 he published his own 

 provitional hypothesis of Pangenesis. But an examma- 

 tion of ihe bearings of the two hypotheses shows that the 

 former does not render the latter superfluous, nor is the 

 one inconsistent with the other. Mr. Darwin wished to 

 picture to himselt and to enable others to picture to them- 

 selves a process which would account for (that is, hold 



together and explain) not merely the simpler facts of 

 hereditary transmission, but those very curious though 

 abundant cases in which a character is transmitted in a 

 latent form and at last reappears after many generations, 

 such cases being known as " atavism " or '* reversion ; " 

 and again those cases of latent transmission in which 

 characteristics special to the male are transmitted to the 

 male offspring through the female parent without being 

 manifest in her ; and yet again the appearance at a par- 

 ticular period of life of characters inherited and remaining 

 latent in the young organism. According to the hypo- 

 thesis of pangenesis, " every unit or cell of the body 

 throws off gemmules or undeveloped atoms, which are 

 transmitted to the offspring of both sexes and are multi- 

 plied by self-division. They may remain undeveloped 

 during the early years of life or during successive genera- 

 tions ; their development into units or cells, like those 

 from which they were derived, depending on their affinity 

 for, and union with, other units or cells previously deve- 

 loped in the due order of growth." 



In an essay ("Comparative Longevity," Macmillan, 

 1870, p. 32) published six years ago, I briefly suggested 

 the possibility of combining Mr. Herbert Spencer's and 

 Mr. Darwin's hypotheses thus : " The persistence of the 

 same material gemmule and the vast increase in the 

 number of gemmules, and consequently of material bulk,^ 

 make a material theory difficult. Modified force-centres, 

 becoming further modified in each generation, such as 

 Mr. Spencer's physiological units, might be made to fit in 

 with Mr. Darwin's hypothesis in other respects." In fact 

 in place of the theory of emission from the constituent 

 cells of an organism of material gemmules which circu- 

 late through the system and affect every living cell, and 

 accumulate in sperm-cells and germ-cells, we may substi- 

 tute the theory of emission of force, the two theories 

 standing to one another in the same relation as the emis- 

 sion and undulatory theories of light. 



It may, however, be very fairly questioned whether our 

 conceptions of the vibrations of complex molecules, or in 

 other words their force-affections, are sufficiently advanced 

 to render it desirable to substitute the vaguer though pos- 

 sibly truer undulatory theory of heredity for the more 

 manageable molecular theory (Pangenesis). How are we 

 to conceive of the propagation of such states of force- 

 affection or vibration (as they are vaguely termed) through 

 the organism from unit to unit ? In what manner, again, 

 are we to express the dormancy of the pangenetic gem- 

 mules in terms of molecular vibration .? It is true that 

 molecular physics furnishes us with some analogies in the 

 matter of the propagation of particular states of force- 

 affection from molecule to molecule, as, for example, in 

 the various modes of decomposition exhibited by gun- 

 cotton, in contact actions and the like'; but it will require 

 a very extended analysis of both the phenomena of 

 heredity and of molecular phenomena similar to those 

 just cited, to enable us to supersede the admittedly pro- 

 visional hypothesis of Pangenesis by a hypothesis of 

 vibrations. And it is necessary here to remark that in 

 the fundamental conception of Pangenesis, namely, the 

 detachment from the living cells of the organism of gem- 

 mules which then circulate in the organism, there is 

 nothing contrary to analogy, but rather in accordance 

 with it. It is quite certain that in some infective diseases 

 the contagion is spread by specific material particles. 

 This seems to be established, although it is far from 

 settled as to whether these particles are parasitic organ- 

 isms or portions of the diseased organism itself. Mr. 

 Darwin's pangenetic gemmules may, even if not accumu- 

 lated and transmitted from generation to generation, be 

 called upon to explain the solidarity of the constituent 

 cells of one organism ; they may be assumed as agents of 



\ On this subject see Mr. Sorby's recent Presidential Address to the Royal 

 Microscopical Society, in " Quarterly Journal of Microscopical Science," 

 April, 1876. 



