242 



HARDWICKE'S SCIENCE-GOSSIP. 



2. Mr. Darwin's " Provisional hypothesis of Pan- 

 genesis" ("Variation of Animals and Plants under 

 Domestication," vol. ii. 186S). 



3. Mr. Francis Gallon's " stirp " and " gemmule " 

 theory, (" Proc. Roy. Soc," No. 136, 1872, and 

 " Journ. Anthrop. Inst,," vol. v. 1876). 



4. Professor Hackel's " Perigenesis of a Plastidule" 

 (" Ueber de Wellenzeugung der Lebenstheilchen," 

 1S76). 



5. Professor Njigeli's "idioplasm" and "germ" 

 theory (" Mechanisch-physiologische Theorie der 

 Abstammungslehre," 1S84). 



6. Professor Weismann's theory of the " Con- 

 tinuity of the Germ plasm " (1885). 



This does not profess to be a complete list of 

 theories of heredity, but it contains the most repre- 

 sentative and important contributions to the subject. 



Mr. Herbert Spencer's and Professor Hackel's 

 theories (which are practically identical) must however 

 be treated separately from the rest. These two 

 authors aim at arriving at general conceptions about 

 the nature of the qualities possessed by the bearers of 

 hereditary tendencies which enable the latter to build 

 up an organism like the parent. They do not give us 

 any particular and definite information on either the 

 process of transmission or ihe process of development : 

 and hence their hypotheses do not admit of the 

 analysis indicated above. 



Mr. Spencer supposes the existence of certain 

 "physiological units," * of which the protoplasm of 

 a cell is composed. They are the ultimate biological 

 units, just as the molecule is the ultimate physical unit, 

 and the atom the ultimate chemical unit. They 

 possess "polarities " which cause them to build up an 

 organism of a definite shape, just as the polarities of 

 the molecules of a crystal cause the latter to build up 

 a crystal of a definite shape. They are made up of, 

 and are immensely more complex than, the molecules 

 of the chemical substances of which protoplasm is 

 composed, and "in each organism the physiological 

 units produced by this further compounding of highly 

 compound atoms [molecules] have a more or less 

 distinctive character, f We must conclude that in 

 each case, some slight difference of composition in 

 these units, leading to some slight difference in their 

 mutual play of forces, produces a difference in the 

 form which the aggregate of them assumes." J The 

 sperm-cells and egg-cells are simply "vehicles in 

 which are contained small groups of the physiological 

 units in a fit state for obeying their proclivity towards 

 the structural arrangement of the species they belong 

 to." § Any modification of the function or structure 

 of any part of an organism will cause, "some cor- 



* Professor Michael Foster ("Text-Book of Physiology," 

 f fth edition, Jntroduction, pp. 5, 6) has adopted this conception 

 in order to explain the facts of metabolism, and the diflerences 

 of different tissues. He differs from Mr. Spencer in this last 

 part of the hypothesis. 



+ Cf. the "idioplasm" of Niigcli. 



J " Principles of Biology," vol. i. p. 183. 



5 Ibid. p. 254. 



responding modification " of " the structures and 

 polarities of its units." These modified polarities will 

 tend to re-establish equilibrium between the forces of 

 the aggregate and those of the unit, and hence 

 separated groups of units will tend to build up an 

 organism modified in a similar manner to the one from 

 which they were derived. Thus does Mr. Spencer 

 attempt to give us a conception of the manner in which 

 he imagines both ordinary inheritance and also the 

 inheritance of acquired modification, can take place. 



Professor Hackel substitutes "plastidule" for 

 "physiological unit," and "vibrations" or " undu- 

 latory movements " for " polarities " ; but apart from 

 these changes of terminology, his hypothesis appears 

 to be substantially the same as that of Mr. Spencer, 

 which, however, he seems not to have read."' 



Various criticisms can be obviously made. In the 

 first place a living organism is not a crystal, nor do its 

 ultimate living constituents — call them physiological 

 units or protoplasmic molecules or what you will — 

 act in an exactly similar way to the molecules of the 

 latter ; and although Mr. Spencer carefully avoids 

 stating that they do, this is what he practically 

 assumes in his explanation of normal heredity and 

 the repair of lost parts. But he also states that a 

 modification of structure or function causes modified 

 structure and polarities of the physiological units, and 

 that in consequence separated groups of the latter 

 build up a similarly modified organism. But if this is 

 sometimes the case, why is there no hereditary effect 

 in the case of the loss of those parts of organisms 

 which are normally repaired ? such as the leg of a 

 newt, or the tail of a lizard or of a tadpole. There is 

 surely a sufficient modification here to " impress some 

 corresponding modification on the structures and 

 polarities " of the physiological units. And on the 

 former assumption, why are lost parts not always 

 repaired ? Why does not a man grow a new leg 

 when he has lost one ? Where is the difference be- 

 tween the two sets of cases ? It has never (on Mr. 

 Spencer's theory) been pointed out. As a matter of 

 fact, the two explanations are mutually exclusive. 

 x\nd even supposing they were not, they both depend 

 upon the unjustifiable assumption of absolute solida- 

 rity between the parts of an organism. No doubt 

 the relations of the different parts of the body to each 

 other and to the whole are highly important, but it 

 is impossible to maintain that a slight modification of 

 any given part necessarily aftects permanently every 

 other part ; and this is what Mr. Spencer's assump- 

 tion practically amounts to. And we have never 

 been furnished with a clear and definite account of 

 the mechanism by which the effects of such modifica- 

 tions arc propagated through the body to distant 

 cells — cells, too, which are often fully developed and 

 have become perfectly quiescent years before. This 



* I am not acquainted with Professor Hackel's hypothesis at 

 first hand. 



