528 



NATURE 



\_Sept. 26, 1889 



the reproductive cells. From his point of view the structural or 

 other properties which characterize a family, a race, or a species 

 are derived solely from the reproductive cells through continuity 

 of their germ-plasm, and are not liable to modification by the 

 action on them of the organs or tissues of the body of the indi- 

 vidual organism in which they are situated. To return for one 

 moment to my graphic illustration in elucidation of this part of 

 the theory. The cells which make up the personal structure of 

 A or B would exercise no effect upon the character of the re- 

 productive cells a or ab contained within them. These latter 

 would not be modified or changed in their properties by the 

 action of the individual organism A or R. The individual B 

 would be in hereditary descent, not from K -k- a, but only from 

 a, with which its germ-plasma ab would be continuous, and 

 through which the properties of the family, race, or species 

 would be transmitted to C, and so on to other successive 

 generations. 



The central idea of Heredity is permanency ; that like begets 

 like, or, as Mr. Galton more fitly puts it, that "like /t'«^/j- to pro- 

 duce like." But though the offspring conform with their parents 

 in all their main characteristic-:, yet, as everyone knows, the child 

 is not absolutely like its parents, but possesses its own character, 

 its own individuality. It is easy for anyone to recognize that 

 differences exist amongst men when he compares one individual 

 with another : but it is equally easy for those who make a special 

 study of animals to recognize individual differences in them also. 

 Thus a pigeon or canary fancier distinguishes without fail the 

 various birds in his flock, and a shepherd knows every sheep 

 under his charge. But the anatomist tells us that these differ- 

 ences are more than superficial — that they also pervade the 

 internal structure of the body. In a paper which I read to the 

 meeting of this Association in Birmingham so long ago as 1865,^ 

 after relating a series of instances of variation in structure 

 observed in the dissections of a number of human bodies, I 

 summarized my conclusion as follows : " Hence, in the develop- 

 ment of each individual, a morphological specialization occurs 

 both in internal structure and external form by which distinctive 

 characters are conferred, so that each man's structural in- 

 dividuality is an expression of the sum of the individual 

 variations of all the constituent parts of his frame." 



As in that paper I was discussing the subject only in its 

 morphological relations, I limited myself to that aspect of the 

 question ; but I might with equal propriety have also extended 

 my conclusion to other aspects of man's nature. 



Intimately associated, therefore, with the conception of 

 Heredity — that is, the transmission of characters common to 

 both parent and offspring — is that of Variability — that is, the 

 appearance in an organism of certain characters which are 

 unlike those possessed by its parents. Heredity, therefore, may 

 be defined as the perpetuation of the like ; Variability, as the 

 production of the unlike. 



And now we may ask, Is it possible to offer any feasible 

 explanation of the mode in which variations in organic structure 

 take their rise in the course of development of an individual 

 organism ? Anything that one may say on this head is of course 

 a matter of speculation, but certain facts may be adduced as 

 offering a basis for the construction of an hypothesis, and on 

 this matter Prof. Weismann makes a number of ingenious 

 suggestions. 



Prior to the conjugation of the male and female pronuclei to 

 form the segmentation nucleus a portion of the germ-plasm is 

 extruded from the egg to form what are called the polar bodies. 

 Various theories have been advanced to account for the signi- 

 ficance of this curious phenomenon. Weismann explains it on 

 the hypothesis that a reduction of the number of ancestral germ- 

 plasms in the nucleus of the egg is a necessary preparation for 

 fertilization and for the development of the young animal. He 

 supposes that by the expulsion of the polar bodies one-half the 

 number of ancestral germ-plasms is removed, and that the 

 original bulk is restored by the addition of the male pronucleus 

 to that which remains. As precisely corresponding molecules of 

 this plasm need not be expelled from each ovum, similar ances- 

 tral plasms lare not retained in each case ; so that diversities 

 would arise even in the same generation and between the offspring 

 of the same parents. 



Minute though the segmentation nucleus is, yet microscopic 

 research has shown that it is not a homogeneous structureless 

 body, but is built up of different parts. Most noteworthy are 



' Transactions of Sections, p. iii, 1865, and Trans. Roy. Soc. Edinburgh, 

 vol. xxiv., 1865. 



the presence of extremely delicate threads or fibrils, called the 

 chrotnatin filaments, which are either coiled on each other, or 

 intersect to form a network-like arrangement. In the meshes of 

 this network a viscous — and, so far as we yet know, structureless 

 — substance is situated. Before the process of division begins 

 in the segmentation nucleus these filaments swell up and then 

 proceed to arrange themselves at first into one and then into two 

 star-like figures before the actual division of the nucleus takes 

 place. ^ It is obvious, therefore, that the molecules which enter 

 into the formation of the segmentation nucleus can move within 

 its substance, and can undergo a readjustment in size and form 

 and position. But this readjustment of material is, without 

 doubt, not limited to those relatively coarse particles which can 

 be seen and examined under the microscope, but applies to the 

 entire molecular structure of the segmentation nucleus. Now it 

 must be remembered that the cells of the embryo from which all 

 the tissues and organs of the adult body are derived are them- 

 selves descendants of the segmentation nucleus, and they will 

 doubtless inherit from it both the power of transmitting definite 

 characters and a certain capacity for readjustment both of their 

 constituent materials and the relative positions which they may 

 assume towards each other. One might conceive, therefore, 

 that if in a succession of organisms derived from common 

 ancestors the molecular particles were to be of the same com- 

 position and to arrange themselves in the segmentation nucleus 

 and in the cells derived from it on the same lines, these successive 

 generations would be alike ; but if the lines of adjustment and the 

 molecular constitution were to vary in the different generations, 

 then the products would not be quite the same. Variations in 

 structure, and to some extent also in the construction of parts, 

 would arise, and the unlike would be produced. 



In this connection it is also to be kept in mind that in the higher 

 organisms, and, indeed, in multicellular organisms generally, anin- 

 dividual is derived, not from one parent only, but from two parents. 

 Weismann emphasizes this combination as the cause of the pro- 

 duction of variations and the transmission of hereditary individual 

 characters. If the proportion of the particles derived from each 

 parent and the forces which they exercise were precisely the 

 same in any individual case, then one could conceive that the 

 product would be a mean of the components provided by the 

 two parents. But if one parent were to contribute a larger 

 proportion than the other to the formation of a particular 

 organism, then the balance would be disturbed, the offspring in 

 its character would incline more to one parent than to the other, 

 according to the proportion contributed by each, and a greater 

 scope for the production of variations would be provided. These 

 differences would be increased in number in the course of 

 generations, owing to new combinations of individual characters 

 arising in each generation. 



As long as the variations which are produced in an organism 

 are collectively within a certain limitation, they are merely 

 individual variations, and express the range within which such 

 an organism, though exhibiting differences from its neighbours, 

 may yet be classed along with them in the same species. It is in 

 this sense that I have discussed the term Variability up to the 

 present stage of this address. Thus all those varieties of mankind 

 which, on account of differences in the colour of the skin, we 

 speak of as the v/hite, black, yellow races and red-skins are men, 

 and they all belong to that species which the zoologists term 

 Hovio sapiens. 



But the subject of Variability cannot, in the present state of 

 science, be confined in its discussion to the production of indi- 

 vidual variations within the limitations of a common species. 

 Since Charles Darwin enunciated the proposition that favourable 

 variations would tend to be preserved, and unfavourable ones to 

 be destroyed, and that the result of this double action, by the 

 accumulation of minute existing differences, would be the for- 

 mation of new species by a process of natural selection, this 

 subject has attained a much wider scope, has acquired increased 

 importance, and has formed the basis of many ingenious specu- 

 lations and hypotheses. As variations, when once they have 

 arisen, may be hereditarily transmitted, the Darwinian theory 

 might be defined as Heredity modified and influenced by 

 Variability. 



This is not the place to enter on a general discussion of the 

 Darwinian theory, and even if it were, the time at our disposal 



' The observations more especially of Flemming, E. Van Beneden, Stras- 

 burger, and Carnoy maybe referred to in connection with the changes which 

 take place in nuclei prior to and in connection with their division. 



