XXXVI 



of each new cell and every change which it causes, certainly takes place 

 suddenly, but what the theory means is the spontaneous appearance of changed 

 forms, not as the expression of a process of development. On the other hand, a 

 few days ago I read an article in the North American Review of Sept. 19 14, 

 entitled ^Thc Evohdion of the chiri in which Dr. Louis Robertson carefully 

 follows the evolution of this part of the human body, and shows how in 

 connection with the development of articulate speech, it has gradually, not 

 suddenly, developed. 1 am well acquainted with apparent mutations. The full 

 grown larva of Papilio Memnon L. is strikingly different in shape from the 

 form it bears before its final shedding of the skin ; the new form in this case 

 appears suddenly. But the different forms of the caterpillar are only the repetition, 

 in accordance with the so-called biological law of Haeckel, of its previous forms, 

 which must originally have come about gradually, but which now, in a still quite 

 unexplained manner, are concentrated in the periods divided by each shedding of 

 the skin and after each moulting appear as a sudden change. This suddenness 

 is, however, only apparent, the process of which it is an expression is by no 

 means clear to us, but for all that it exists indubitably. As a matter of fact 

 the last shape of this caterpillar is not an isolated form entirely disconnected 

 with its earlier ones; this is shown clearly by the larvae of related species, 

 which have gone through the same process of change, but not so completely, 

 retaining therefore in various respects a resemblance to the older forms, which 

 has completely disappeared in the Papilio Memnon L. caterpillar. Is it not 

 possible, that in the field of Botany there may be obscure processes which 

 cause what seem to be sudden mutations, although in reality this is a false 

 appearance? My observations have led me to the conviction, that each organ, 

 or rather each physiological unit, can become subject to a pressure towards 

 spontaneous changes, for the development of which in the individual a special 

 succeptibility — the conditon called in the mutation theory a "mutation period" — 

 is necessary ; which susceptibility does not arise however at the same time and 

 in the same degree in every individual; the consequence of which may be, 

 that between the individuals of one species in whom the susceptibility is present 

 and those in which it is absent, a division may arise, whereby a new species 

 comes into existence. Where a susceptibility of this kind has arisen, it may 

 also appear in other physiological units; or correlation may cause this and 

 when by these means a further impulse towards change has become active, 

 naturally the separation may assume greater proportions. These processes 

 develop very differently according to the greater or less susceptibility of the 

 individual and the period of its development, and therefore proceed with greater 

 or less rapidity. The true nature of this susceptibility is not known; some of 



