348 Professor J. Arthur Thomson [March 30, 



We know of twin animals developed from one egg, but what shall 

 we say of the quadruplets Wilson obtained by shaking apart the 

 four-cell stage in the development of the lancelet, or of the " legion 

 of embryos " which Marchal describes as developing from a single 

 ovum of a peculiar Hymenopterous insect Encyrtus f In development, 

 indeed, a half may be as good as a whole. 



In reference to the difficulty raised in some minds by the minute- 

 ness of the physical basis, it may be recalled that the students of 

 physics, who make theories regarding the sizes of atoms and molecules, 

 which they have invented, tell us that the image of a Great Eastern 

 filled with framework as intricate as that of the daintiest watches 

 does not exaggerate the possibilities of molecular complexity in a 

 spermatozoon, whose actual size may be less than the smallest dot on 

 the watch's face. Secondly, as we learn from embryology that one 

 step conditions the next and that one structure grows out of another, 

 we are not forced to stock the microscopic germ-cells with more than 

 initiatives. Thirdly, we must remember that the development im- 

 plies an interaction between the growing organism and a complex 

 environment without which the inheritance would remain unexpressed, 

 and that the full-grown organism includes much that was not inherited 

 at all, but has been acquired as the result of nurture or external 

 influence. 



The central problem of heredity is to form some conception of 

 what we have called the relation of genetic continuity between 

 successive generations ; the central problem of inheritance is to 

 measure the resemblances and differences in the hereditary characters 

 of successive generations, and to arrive, if possible, at some formula, 

 which will sum up the facts. It is inexpedient to lay on the shoulders 

 of the student of heredity the burden of problems, which are not in 

 any special sense his business. It is no doubt interesting to ask how 

 an organisation supposed to be very complex, may be imagined to find 

 physical basis in a microscopic germ-cell, but the same sort of question 

 may be raised in regard to a ganglion-cell. It is not distinctively a 

 problem of heredity. It is interesting to inquire into the orderly 

 and correlated succession of events by which the fertilised egg-cell 

 gives rise to an embryo, but this is the unsolved problem of physio- 

 logical embryology. It raises questions distinct from those of heredity 

 and inheritance, and apparently much less soluble. 



In the preformationist theories, which held sway in the seven- 

 teenth and eighteenth centuries — theories which asserted the pre- 

 existence of the organism and all its parts, in miniature, within the 

 germ — there was a kernel of truth well concealed within a thick 

 husk of error. For we may still say, as the preformationists did, 

 that the future organism is implicit in the germ, and that the 

 germ contains not only the rudiment of the adult organism, but the 

 potentiality of successive generations as well. But what baffled the 

 earlier investigators was the question how the germ-cell comes to 

 have this ready-made organisation, this marvellous potentiality. 



