676 



HEREDITY 



mission of individually acquired characters. Be it 

 clearly understood that ' natural inheritance ' is a 

 certain fact ; innate, constitutional, congenital, or 

 germinal qualities, and the results of these in the 

 parents, are certainly transmissible to the offspring ; 

 the disputed problem, which awaits experimental 

 evidence, is to what degree, if any, extrinsic, func- 

 tional, or environmental modifications acquired by 

 the parents can be handed on as a legacy for good 

 or ill to the offspring. That such acquisitions often 

 recur is indubitable, but it is not at present certain , 

 that they recur because they have been transmitted. 

 They may of course be the result of the action on 

 the offspring of the same conditions as first evoked 

 them in the parents. 



Problems. In regard to the relation between 

 parents and offspring, there are three great pro- 

 blems to be discussed. What is the peculiarity of 

 the germ-cells which enables them (in most cases 

 after uniting as male and female elements) to 

 develop into organisms essentially like the parents ? 

 Granting that the germ-cells are in some respects 

 unique when compared with the ordinary cells of 

 the ' body,' granting that the fertilised egg is in some 

 sense a potential organism, how are we to think 

 of the mechanism of development by which the 

 specific type is reconstructed ? Thirdly, what is 

 the probable truth, or present state of opinion, in 

 regard to the transmission of acquired as opposed 

 to constitutional or germinal characters ? In addi- 

 tion to these three great problems of individual 

 inheritance, there are minor questions in regard 

 to atavism, reparation of injuries, and the like, 

 detailed practical inquiries as to the inheritance of 

 disease, and, widest of all, those pi-oblems of social 

 inheritance which concern the relation between 

 large fraternities of the human species through 

 successive generations. 



Mystical Theories. Theories of heredity, like 

 those of many other facts, have been expressed in 

 three sets of terms theological, metaphysical, and 

 more or less scientific. The ancient hypotheses, 

 that germs were possessed and controlled by spirits, 

 gave place to theories which invoked ' principles of 

 heredity ' and ' formative forces,' and these in turn 

 have been displaced by more concrete conclusions. 

 Of most historical importance are the so-called 

 'mystical' or ' preformation theories,' according to 

 which the male or female germ contained a minia- 

 ture model of the future organism, and indeed of 

 succeeding generations as well, while the develop- 

 ment was merely a gradual unfolding or literal 

 'evolution.' We still believe of course that the 

 fertilised egg is a potential organism, and that it 

 has great complexity within its apparent simplicity, 

 but the researches of the founders of embryology 

 were enough to show that no miniature models 

 existed, and that development was anything but the 

 unfolding of a bud. See EMBRYOLOGY. 



Pangenesis. Many naturalists have attempted to 

 explain the uniqueness of the germs or germ-cells by 

 regarding them as concentrations of units collected 

 from the various structures of the body. The hypo- 

 thetical process by which these units are given off 

 from the various organs, travel to the seat of the 

 germs, and are there accumulated to reproduce in 

 the embryo structures like those whence they 

 originated, is termed pangenesis. At such different 

 epochs as are suggested by the names of Democritus 

 and Hippocrates, Paracelsus and Buffon, pangenetic 

 theories were advanced. The first clear theory, 

 however, was that of Spencer ( 1864), who suggested 

 the existence of 'physiological units,' derived from 

 and capable of development into cells, and supposed 

 their accumulation in the reproductive elements. 

 But the best-known form of the theory is Darwin's 

 'provisional hypothesis of pangenesis' (1868), 

 according to which (a) every cell of the body, not 



too highly differentiated, throws off characteristic 

 gemmules, which (b) multiply by fission, retaining 

 their peculiarities, and ( c ) become specially concen 

 tratea in the reproductive elements, where (c?) in 

 development they grow into cells like those from 

 which they were originally given off. Somewhat 

 later (1876) the ingenious Jager sought to replace 

 Darwin's gemmules by characteristic ' scent-stuffs,' 

 which were collected from the body into the repro- 

 ductive elements ; he suggested, in other words, 

 what may be called chemical pangenesis. Mean- 

 while ( 1872) Galton had been led by his experiments 

 on the transfusion of blood and by other consider- 

 ations to the conclusion that ' the doctrine of pan- 

 genesis, pure and simple, is incorrect.' While 

 reaching forward to something better, he still 

 allowed a limited pangenesis to account for those 

 cases which suggest at least that acquired characters 

 are 'faintly heritable.' He admitted that a cell 

 'may throw off a few germs [i.e. gemmules] that 

 find their way into the circulation, and have thereby 

 a chance of occasionally finding their way to the 

 sexual elements, and of becoming naturalised among 

 them.' In 1883 Professor W. K. Brooks proposed 

 an important modification of Darwin's theory, 

 especially insisting on the following three points : 

 that it is in unwonted and abnormal conditions that 

 the cells of the body throw off gemmules ; that the 

 male elements are the special centres of their accu- 

 mulation ; and that the female cells keep up the 

 feneral resemblance between offspring and parent, 

 'or criticism of the numerous suppositions involved 

 in the various theories of pangenesis, the reader is 

 referred to the works of Galton, Ribot, Brooks, 

 Herdman, Plarre, De Vries, and others (see biblio- 

 graphy) ; it is enough for our purpose to notice, in 

 the light of the next step of progress, the compara- 

 tive gratuitousness of any such special theories. 



Fact of Continuity. As far back as 1849 Owen 

 pointed out that in the developing germ it was 

 possible to distinguish between those cells which 

 became much changed to form the 'body,' and 

 those which remained virtually unchanged and 

 formed the reproductive organs. The same distinc- 

 tion was emphasised by Haeckel and by Rauber, 

 while Jager expressed his views very explicitly as 

 follows : ' Through a great series of generations 

 the germinal protoplasm retains its specific pro- 

 perties, dividing in development into a portion 

 out of which the individual is built up, and a 

 portion which is reserved to form the reproductive 

 material of the mature offspring.' This reserva- 

 tion, by which the germinal protoplasm is sheltered 

 from external or corporeal influences, and retains 

 its specific and embryonic characters unchanged 

 from the parent ovum, Jager called by a now famous 

 phrase 'the continuity of the germ -protoplasm.' 

 Brooks (1876, 1877, 1883) was not less clear: 

 ' The ovum gives rise to the divergent cells of the 

 organism, but also to cells like itself. The ovarian 

 ova of the offspring are these latter cells, or their 

 direct unmodified descendants. The ovarian ova of 

 the offspring thus share by direct inheritance all the 

 properties of the fertilised ova.' In the same way 

 Galton (1872, 1875), using the term 'stirp' to ex- 

 press the sum total of germs, gemmules, or organic 

 units of some kind in the fertilised ovum, main- 

 tained that a certain residue is kept apart from 

 the development of the body, to form the repro- 

 ductive elements of the offspring. The history 

 must also include Nussbaurn, who likewise called 

 attention to the very early differentiation and 

 isolation of the sex-elements to be observed in 

 the development of some animals. The general 

 notion independently suggested by the above 

 naturalists is simple enough. At an early stage 

 in the development of the embryo the future re- 

 productive cells of the organism are distinguishable 



