IV.] FOUNDATION OF A THEORY OF HEREDITY. 1 75 



writers started with the hypothesis that there must be a direct 

 connexion between the germ-cells of succeeding generations, 

 and they tried to establish such a continuity by supposing that 

 the germ-cells of the offspring are separated from the parent 

 germ-cell before the beginning of embryonic development, or 

 at least before any histological differentiation has taken place. 

 In this form their suggestion cannot be maintained, for it is in 

 conflict with numerous facts. A continuity of the germ-cells 

 does not now take place, except in very rare instances ; but 

 this fact does not prevent us from adopting a theory of the 

 continuity of the germ-plasm, in favour of which much weighty 



common group and converge to a common contribution, because they 

 were both evolved out of elements contained in a structureless ovum, 

 and they, jointly, contribute the elements which form the structureless 

 ova of their offspring.' The following diagram shows clearly 'that the 

 span of each of the links in the general chain of heredity extends from 

 one structureless stage to another, and not from person to person : — 



Structureless elements { . . Adult Father . . ) structureless elements 



in Father ( . . Latent in Father . .) in Offspring.' 



Again Galton states — ' Out of the structureless ovum the embryonic 

 elements are taken . . . and these are developed {a) into the visible adult 

 individual; on the other hand . . ., after the embryonic elements have 

 been segregated, the large residue is developed {b) into the latent 

 elements contained in the adult individual.' The above quoted sentences 

 and diagram indicate that Galton does not derive the whole of the 

 hereditary tendencies from the latent elements, but that he believes 

 some effect is also produced by the patent elements. When however 

 he contrasts the relative power of these two influences, he attaches 

 comparatively little importance to the patent elements. Thus if any 

 character be fixed upon, Galton states that it 'may be conceived (i) as 

 purely personal, without the concurrence of any latent equivalents, (2) 

 as personal but conjoined with latent equivalents, and (3) as existent 

 wholly in a latent form.' He argues that the hereditary power in the 

 first case is exceedingly feeble, because ' the effects of the use and 

 disuse of limbs, and those of habit, are transmitted to posterit3' in only a 

 very slight degree.' He also argues that many instances of the supposed 

 transmission of personal characters are really due to latent equivalents. 

 ' The personal manifestation is, on the average, though it need not be so 

 in every case, a certain proof of the existence of latent elements.' 

 Having argued that the strength of the latter in heredity is further 

 supported by the facts of reversion, Galton considers it is safe to 

 conclude ' that the contribution from the patent elements is very much 

 less than from the latent ones.' In the later development of his theory, 

 Galton adheres to the conception of ' gemmules ' and accepts Darwin's 

 views, although Svith considerable modification.' Together with pan- 

 genesis itself, Galton's theory must be looked upon as preformation al, 

 and so far it is in opposition to Weismann's theory which is epigenetic. 

 See Appendix IV, to the next Essay (V). — E. B. P.] 



