June 29, 1900.] 



SCIENCE 



969 



history of an animal, all those factors in its 

 development which did not accord with the 

 theory, were apt to seem to him to be falsi- 

 fications of the record. Another observer 

 with the same facts before him, but work- 

 ing on a different theory, would discover 

 that many of these so-called falsifications 

 were really ancestral features. 



Another factor which has hampered em- 

 bryology as a phylogeuetic discipline has 

 been the too frequent limitation of the in- 

 vestigation to a single organ. It is easier 

 to investigate a single organ through a 

 series of embryos than to investigate the 

 entire structure of all the members of the 

 series. We are able to judge correctly of 

 the character of a man only when we know 

 all the elements that make it up. And so 

 with a series of embryos, we must know the 

 whole structure, not merely a part of it. 

 Monographic work is here quite as neces- 

 sary as in comparative anatomy. 



Many illustrations might be given of the 

 grotesque results reached in animal geneal- 

 ogy, principally through too great reliance 

 on embryology. That investigators with 

 the same facts before them may reach dia- 

 metrically opposite conclusions is shown in 

 the attempt to trace the ancestry of the 

 vertebrates. No less than a dozen inverte- 

 brate groups have been announced from 

 time to time as having furnished the verte- 

 brate ancestor. The ccelenterates, the an- 

 nelids, the nemertines, the Crustacea, the 

 spiders, Balanoglossus and the tunicates 

 have all been candidates for this honor, 

 and perhaps all deserve it equally. 



With such results the zoological pendu- 

 lum may be said to have reached, for the 

 present, the limit of its excursion in the di- 

 rection of phylogeny. It is now beginning 

 to swing in another direction. Within the 

 last five years, zoologists have begun to see 

 that phylogeuetic speculations have been to 

 a large extent fruitless of specific results. 

 They cannot be undertaken to advantage 



until we have vastly widened our field of 

 knowledge. Then too it is being realized 

 that the construction of a phylogeny of ani- 

 mals is, after all, not a matter of the great- 

 est consequence. So long as we know that 

 animals are related to one another and so 

 long as we are able to investigate the laws 

 which have governed the establishment of 

 that relationship, it does not so much matter 

 just ivhat the precise relationship maj' be. 



Zoologists are then turning in other direc- 

 tions. There seem to me to be chiefly four. 



1. There is among those engaged in purely 

 descriptive anatomy or embryology a tend- 

 ency, not yet very pronounced, but yet 

 growing, to return to the monographic 

 method of working. This is a return to the 

 methods of the beginning of the century and 

 betokens a purpose to let speculation rest 

 for a while, until more materials have ac- 

 cumulated upon which to base it. 



2. There is a marked tendency to study 

 variations. The first book on this subject 

 has appeared within a few years, and has 

 stimulated the production of many papers. 

 The purpose of the workers in this field is 

 to determine the nature and range of varia- 

 tion so as to gain a familiarity with the 

 nature of the materials upon which natural 

 selection acts. It may thus be possible, as 

 Bateson points out, for the investigatorof the 

 future to say not ' if such and such a varia- 

 tion should occur,' but ' since such and such 

 a variation does occur.' Students of vari- 

 ation hope also to discover some of the laws 

 which determine the production of varia- 

 tions. It is believed that they are not, as 

 Darwin thought, fortuitous, matters of 

 chance, but that they are subject to well 

 defined laws. 



All phylogeuetic speculation is based 

 upon the idea of homology, but the study 

 of variations has set our ideas of homology 

 toppling and until these ideas are recon- 

 structed we cannot hope for any final deter- 

 mination of animal relationships. 



