NA TURE 



?6i 



THURSDAY, AUGUST 20, 1908. 



uERF.nnx. 



Hcicdify. By Prof. J. Arthur Thomson. Pp. .\vi + 

 605. (London : John Murray, 190S.) Price gs. net. 

 AA ?"£ .nil know hooks on science which we ought to 

 read wilii ple.tsure, but to which we turn witli 

 shrinlving-. Full, perhaps, of new facts and ideas, 

 they are so e.xpressed as to bore consumedly. 

 " Heredity " belongs to another category. He wht) 

 runs may read, even if he be a beginner, and he 

 who reads will probably not cease to run until he has 

 traversed the last page. It contains nothing very 

 new, but most of the facts on which we found our 

 notions of heredity are set out lucidly and in orderly 

 array, as are» almost all the theories ever based on 

 them. It is dedicated to " Francis Galton and August 

 Weismann, whose magistral studies on heredity 

 have made us all their debtors." Prof. Thomson is 

 very loyal to the masters. 



The outstanding feature of the book is its great, 

 perhaps its excessive, kindliness and toleration. 

 Probnblv few biologists agree so thoroughly with so 

 many of their fellows as the author. Practically the 

 only hard things he has to sa\' are about " hereditary 

 tendencies " and " principles of heredity," which seem 

 to him " in part the old story of explaining the 

 working of the clock bv ' principle of horologitv ' and 

 in part a pedantic way of saying ' we don't know.' ' 

 But for many years no serious student of science has 

 used the word principle except as a synonym for 

 that brief and comprehensive summary of facts which 

 is otherwise termed a " law." We speak of the 

 " principles of psychology," the " principles of 

 geology," and so forth; and surely the germ of a 

 mouse develops into a mouse, and not into a beetle, 

 because it has, among other things, a tendencv, a 

 " predisposition " to do so. " Predisposition " is, 

 oddly enough, approved by Prof. Thomson, who pro- 

 nounces predispositions to be " mysterious " but not 

 "mystical." No doubt they are quite as mysterious 

 but not more mystical than eyes and noses. . 



The book has many merits, not the least of which 

 are its comprehensiveness and literary charm. Its 

 principal defect is lack of that " rigorous deductive 

 inference of consequences " by means of which we 

 link together and test hypotheses and so ascertain 

 whether they are in harmony with one another and 

 "with the conceived system of reality." It is not 

 sufficiently critical. Theories, even when incom- 

 patible, are set out with an appreciation that is equally 

 cordial. The law of ancestral inheritance, the theorv 

 of the continuity of the germ-plasm, and the theorv 

 of recapitulation are conspicuous examples. A statis- 

 tical inquiry led Galton to the conclusion that, on the 

 average, progeny resemble progenitors in certain 

 degrees, whence he drew the deduction that the 

 lieritage of descendants is compounded in the stated 

 proportions of ancestral contributions. I have ever 

 been doubtful of the precise meaning of the term 

 "contribution," but apparently it implies more than 

 NO. 2025, VOL. 78] 



mere resemblance, or it would not be used. Readers 

 of " Heredity " will be sure to conceive a contribution 

 as an actual something contributed to the germ-plasm 

 by the progenitor. 



Passages like the following abound : — " \\'e know 

 . . . that the parental heritages include ancestral 

 contributions which may be expressed in development 

 or lie latent." But this notion will be hard to recon- 

 cile with the idea that " the parent is rather the 

 trustee of the germ-plasm than a producer of the 

 child." If, however, readers elect to regard 

 " contribution " as synonymous with ^^'eismann's 

 " ancestral plasm " (a collection of determinants 

 similar to that which controlled the development of 

 the ancestor), they will be puzzled to reconcile the 

 statement that " an individual inheritance is a mosaic 

 of parental and ancestral plasms " with the theory 

 that " the individual development, especiallv in thi; 

 stage of organ-forming, is in some measure a re- 

 capitulation of the racial history," a theorv which 

 represents ancestral contributions, not in the form of 

 a mosaic, but in that of a series. Must we assume, 

 then, that the remote ancestors, in whom the organs 

 were evolved, contributed to a series, but more recent 

 ancestors to a mosaic; or is it the right view that, 

 since the characters of the organism vary inde- 

 pendently, retrogression in some characters, combined 

 with progression in others, produces, in part at least, 

 the appearance of a mosaic? 



Every generation follows more or less closely in the 

 de\elopmental footsteps of the preceding gencratior. 

 and every progressive variation prolongs development 

 by adding itself to the sum of those already made and 

 preserved during phylogeny. Of course, therefore, 

 development is, with large reservations, a recapitula- 

 tion of the life-history. Preservation {i.e. reproduc- 

 tion) implies recapitulation. Presumably the germ- 

 plasm can lose as well as gain ; that is, failure to 

 complete the recapitulation of the parental develop- 

 ment in any particular may be due to true variation, 

 not merely result from injur}' received during develop- 

 ment as Prof. Thomson seems to imply. Suppose an 

 individual varied in such a way as to lapse the progres- 

 sive, variations of many ancestors ;, then, as regards 

 the character in question, he would be in the position 

 of a more or less remote ancestor. Is this reversion? 

 If not, why not? According to Prof. Thomson only 

 the reappearance of a latent character constitutes 

 reversion. He would find it hard to justify his 

 position. 



He insists, as others have done, that " filial regres- 

 sion has nothing to do with reversion," because 

 " there is a levelling up as well as a levelling down." 

 But the fact, admitted bv him, that cessation of selec- 

 tion tends to be followed by retrogression, indicates 

 that on the whole there is a greater tendency to level 

 down than to level up. In other words, retrogressive 

 variations tend to predominate over progressive varia- 

 tions. If, then, development is modified recapitula- 

 tion, it is plain that filial regression has something to 

 do with reversion. At any rate, the attempt to link 

 the two together is not necessarily due to misunder- 

 standing. Doubtless regression is not always a 



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