April 7, 1905.] 



SCIENCE. 



529 



Without attempting to defend any of the 

 hypotheses mentioned it may here be point- 

 ed out that relatively slight modifications 

 in germinal organization would convert one 

 type into another. 



A distinguishing characteristic of the 

 mutation theory is the recognition of ele- 

 mentary characters or properties vi^hich 

 manifest themselves in many separate parts 

 of the adult, as, e. g., the presence or ab- 

 sence of hairs or certain colors; if muta- 

 tions are germinal the widespread distribu- 

 tion of such characters in the adult are 

 easily explained. Relatively slight modi- 

 fications of the germ, however produced, 

 may lead to profound and widespread 

 modifications of the embryo and adult. I 

 have elsewhere shown reason for believing 

 that the cause of inverse symmetry which 

 occurs regularly among some species and 

 occasionally among all, man included, is to 

 be found in the inverse organization of the 

 egg, and that this inverse organization may 

 be due to the maturation of the egg at op- 

 posite poles in dextral and sinistral forms. 

 This case shows that one of the most re- 

 markable and far-reaching forms of varia- 

 tion with which we are acquainted is the 

 result of relatively slight alterations in the 

 localization of germinal substances in the 

 unsegmented egg. 



One of the principal difficulties in 

 explaining the origin, on evolutionary 

 grounds, of different phyla has been the 

 dissimilar locations of corresponding or- 

 gans or parts. These difficulties are well 

 illustrated by the theories which attempt 

 to derive the vertebrates from the annelids, 

 or from any other invertebrate type. If 

 evolution takes place through transforma- 

 tions of the germ rather than of the adult, 

 it is no more difficult to explain the differ- 

 ent locations of corresponding parts in 

 these phyla than their diiferent qualities. 

 Changes in the relative positions of parts 

 which would be absolutely impossible in the 



adult, may be readily accomplished in the 

 unsegmented egg, as is shown by cases of 

 inverse symmetry. The question is here 

 raised whether some similar sudden altera- 

 tion of germinal organization may not lie 

 at the basis of the origin of new types. 



Mutations: Thomas Dwight, Parkman 

 Professor of Anatomy, Harvard Medical 

 School. 



It has been clear from the beginning that 

 evolution, if it be a power at all, must work 

 either by minute modifications or by more 

 or less sudden changes. Darwinism is es- 

 sentially the doctrine of minute modifica- 

 tions increased by selection and controlled 

 by the survival of the fittest. Darwin in- 

 sisted most strongly on the importance of 

 minute modifications. While holding that 

 'strongly marked variations* might modify 

 a species without the help of any selection 

 at all, he absolutely denied any sudden 

 changes of importance siich as lie at the 

 bottom of the mutation theory. 'Natural 

 selection,' he wrote, 'acts only by the pres- 

 ervation and accumulation of small in- 

 herited modifications'; and he asserted that 

 it would 'banish the belief of the continued 

 creation of new organic beings or of any 

 great and sixdden modifications of their 

 structure. ' 



The mutation theory of sudden jumps 

 and, it may be, of long jumps, is far from 

 new ; but it is de Vries 's merit to be able 

 to show by demonstration what before was 

 only theory. His hypothetical 'pangens' 

 by which the changes are said to be broiight 

 about need not be discussed here. A rad- 

 ical difference between the two theories is 

 this: Darwinism pure and simple is essen- 

 tially fortuitous; it aims in no particular 

 direction, there is no goal; while mutation 

 by producing suddenly a new species, or at 

 least a subspecies, implies the existence of 

 a type and of a law which under certain 

 conditions becomes operative. 



