March 6, 1903.] 



SCIENCE. 



381 



The conclusion is reached, accordingly, that 

 the distinctness of species was just as pro- 

 nounced in the past as it is to-day, and that 

 the idea of species has a definite morphological 

 value. But this distinctness can not have 

 been brought about by successive and pro- 

 miscuous minglings of the germ plasm, by 

 amphimixis ; the role of this has rather been 

 to annul in the course of generations extreme 

 variations, and, granting the limitation of 

 amphimixis to a group of forms by the action 

 of migi-ation, isolation or some other such 

 factor, the result wiU have been the consolida- 

 tion or concentration of certain characters, 

 determined by the environment, and the for- 

 mation of a species. A species, then, is ' a 

 product of individual variation and limitation 

 of crossing, and represents a local departure 

 from the general tendency of development ' ; 

 it is a fixation of one of the rapidly changing 

 pictures produced during a general develop- 

 mental progress. 



What then are the factors which determine 

 the general developmental tendency? Of 

 these Professor Jaekel discusses three, namely, 

 orthogenesis, epistasis and metagenesis, none 

 of which is entirely unfamiliar, although the 

 last two may not be recognizable under their 

 new names. The factor of orthogenesis is 

 essentially the orthogenesis of Eimer and the 

 'Vervollkommnungstrieb' of Nageli, extended, 

 however, so as to include progressive modifi- 

 cations of parts as well as of the entire organ- 

 ism, and to embrace as well retrogressive as 

 progressive modification. As examples of its 

 action there are cited the progressive modi- 

 fications in the structure of the arms in the 

 Melocrinidse and Taxocrinidse, the gradual 

 migration of the anus in the Caryocrinidoe 

 from the lower region of the theca to its 

 upper margin, and the progressive complica- 

 tion of the septal lines in the Ammonitidaa. 



Epistasis is a modified form of the process 

 emphasized by Boas under the name of 

 neotenia, a reversion of a phylum to a modi- 

 fied embryonic condition. Evidence for such 

 a factor is found again among the crinoids, 

 in the apparently reversionary peculiarities ob- 

 servable in certain groups, and also in the 

 Saleniidse and in the Agnostidse among the 



trilobites, whose small number of free body 

 segments is regarded as due to an inhibition 

 of development, rather than as an ancestral 

 character. So too the transition of the 

 AcanthodidsB of the Devonian period, with 

 numerous dermal bones on the head and 

 shoulder girdle and with acrodont teeth, to 

 their Permian descendents which some paleon- 

 tologists have regarded as true selachians, is 

 advanced as a case much to the point, and the 

 discovery of two Paleozoic cyclostomes which 

 show, when compared with the more ancient 

 PalceospondyluSj a marked diminution of 

 osseous material in the skeleton, leads to the 

 supposition that this group of fishes may 

 also have arisen as the result of epistasis. It 

 must be confessed, however, that the morphol- 

 ogist who may have followed Professor Jaekel 

 up to this point with interest, if not with abso- 

 lute confidence, will draw a deep breath when 

 he reads that the author is inclined to regard 

 the entire group of the fishes as degenerated 

 vertebrates, whose watery environment inhib- 

 ited their normal development ' und die For- 

 men namentlich in ihrer Atmung zur Stadien 

 zuriickfiihrte, wie wir sie bei Crustaceen an- 

 treffen.' 



Finally, under the factor of metakinesis 

 there are found the results of what embryolo- 

 gists term cenogenetic modification, for the 

 process is defined as a profound modification 

 of a form in a manner impossible in the adult 

 and only possible in a young stage in which 

 the various organs are not yet histologically 

 specialized and still possess more or less plastic- 

 ity. Examples of the action of this force are 

 again drawn from the crinoids, but these can 

 not, within due limits, be detailed here. 

 Among the echinoids the development of the 

 irregular forms from the regular is regarded 

 as the result of metakinesis, and the 'occur- 

 rence in the Trias of Tiarechinus, with more 

 than two rows of interradial plates, is quoted 

 among other examples of its action. 



Such, in brief, are the ideas which Pro- 

 fessor Jaekel advances in his pamphlet, which, 

 it may be said, is a reprint from the ' Verhand- 

 lungen des Y. Intemationalen Zoologen-Con- 

 gresses.' The ideas are not entirely novel, 

 nor does their exjjosition free the mind of a 



