April 7, 1905.] 



SCIENCE. 



535 



There will also arise, I believe, in a work of 

 this kind, a necessity for distinguishing between 

 the essential characters of a group and those char- 

 acters which are used by the systematist merely to 

 enable students to recognize members of the 

 group. For it seems to me that the essential char- 

 acters of a group of organisms do not lie neces- 

 sarily in the presence or absence of any structure 

 or structures, or in the form or any part or parts 

 of the body of the living members of the group ; 

 but rather in the characteristic structure of the 

 progenitor of the gi'oup, and in the direction of 

 specialization of the descendants of this progenitor. 



The recognition-characters are those usually 

 first observed by the investigator, and are those 

 commonly given in taxonomic works. In many 

 cases these recognition-characters are also essen- 

 tial characters, especially in the case of groups 

 that have been thorougnly studied. But by the 

 taxonomic methods now commonly used, search is 

 chiefly made for recognition-characters. The more 

 skilled the systematist the more likely is he to dis- 

 cover and use as recognition-characters those that 

 are really essential, although the distinction 

 pointed out here may not be recognized by him. 



Very likely we shall not abolish the pres- 

 ent systems of nomenclature and descrip- 

 tion in the larger units, but we shall 

 modify and extend them. We shall break 

 away from the old lines of cleavage. We 

 shall learn what marks that are correlated 

 with function can be used as expedient 

 diagnostic characters. We shall make an 

 increasing effoi't to use absolute characters, 

 not merely relative and comparative ones. 

 We ought to make the 'type' of the species 

 the real biological or phylogenetic type, not 

 cling merely to the ' original ' specimen that 

 chanced first to be named. What we now 

 call 'types' may be wholly unusual and 

 even non-significant forms. If the book or 

 literary type is in time to be the real type, 

 then we shall re-group our species-units, 

 and this will be the greatest possible gain. 



If we decide that literary-species must 

 come, in the future, to correspond to the 

 physiological or elementary species, then 

 we may hope to express the direction of 

 evolution fairly well in our taxonomic 

 schemes. These taxonomic schemes must 



proceed centrifugally and dichotomously 

 rather than lineally. They must arrange 

 about foci. I wish to quote again from 

 Comstock : 



If the history of a group be worked out in the 

 manner indicated, the student will feel the need 

 of recording his results in such a way as to indi- 

 cate the phylogeny of the divisions of the group. 

 But as the necessities of book-making require a 

 linear arrangement of descriptions, this is some- 

 what difficult ; for the natural sequence of groups 

 should be represented by constantly branching 

 lines rather than by a single straight line. 



It seems to me that the most practicable way 

 of meeting this difficulty is to begin with the 

 description of the most generalized form known, 

 and to follow this with descriptions of forms rep- 

 resenting a single line of development, passing 

 successively to more and more specialized forms 

 included in this line. When the treatment of 

 one line of development has been completed, take 

 up another line, beginning with the most gener- 

 alized member of that line and clearly indicating 

 in the text that a new start has been made. 



In making the foregoing suggestions I 

 am well aware that I have not devised 

 any definite nomenclatorial or taxonomic 

 schemes by which they can be carried out. 

 I doubt whether it is worth while to devise 

 any schemes. We need only to establish 

 a few principles and to look upon the pres- 

 ent methods as temporary, allowing new 

 methods to grow as our ideas grow. There 

 can be no finality in such schemes or sys- 

 tems. We have lately seen a vigorous re- 

 vival of the efi^ort towards 'stability' of 

 nomenclature ; but nomenclature is only a 

 bit of language, and language can never be 

 stable if it is vital. It was the old idea 

 that systematic work is for the purpose of 

 making record ; it is the new idea that it is 

 for the purpose of expressing the meaning 

 of the organic creation. 



Ethology and the Mutation Theory: Wil- 

 liam Morton Wheeler, Curator of In- 

 vertebrate Zoolog}% American Museum 

 of Natural History. 



"The mutation theory," .as we learn 



