March 6, 1896.] 



SCIENCE. 



361 



to show that without this principle of isolation 

 differentiation of species is unimaginable, Ort- 

 mann gives his view on the origin of variations 

 and their modification in different species. 

 In regard to the origin of variations he follows 

 those authors which explain variation by direct 

 adaptation of the surroundings (I would prefer 

 to say by the direct influence of the surround- 

 ings). He is entirely opposed to Weismann's 

 idea, according to which variation originates 

 by Amphimixis, i. e. , the union of two elements 

 (germplasmas of different kind). His principal 

 objection is that Weismann, in order to explain 

 the prigin of variation, introduces the principle 

 of Amphimixis, but allows this to operate with 

 material already varied, the difference of the 

 germplasmas. Weismann, in order to explain 

 the origin of differences, takes for granted their 

 preexistence. For this reason alone Amphi- 

 mixis as the source of variation is inadmissible. 

 On the other hand it is known by experience 

 that amphimixis, if operating with different 

 material, will not produce new, but will unite 

 together existing differences, especially if the 

 different material is already similar and closely 

 related. In this connection he refers especially 

 to a paper by Pfeffer : Die inneren Fehler der 

 Weismann' schen Keimplasma-theorie. Ver- 

 handl. Naturw. Ver. Hamburg (3) I., 1894, 

 which seeems to be very little known.* 



The two factors which form the basis for the 

 process of the formation of species are according 

 to Ortmann: 1, the adaptability of the organ- 

 isms themselves to the external conditions, and 

 2, the possibility of the inheritance of the char- 

 acters thus acquired. To these a third factor is 

 added, natural selection. Natural selection oper- 

 ates in such a way that out of the number of 

 existing forms those are exterminated which are 

 unfavorably placed. It is not a selection of the fit, 

 but a destruction of the unfit. By this destruc- 

 tion of the bad individuals the average of the 

 totality is raised; that is, the average of the 



* There are some other papers by G. Pfeffer, which 

 are of great importance in these questioDs: Die Um- 

 wandlung der Arten eiuVorgang funotioneller Selbst- 

 gestaltung. Verhandl. Naturwiss. Verein. Ham- 

 burg (3) I., 1894, 44 pp., and Die Entwicklung. 

 Eine naturwissenscliaftliche Betrachtung. Berlin. 

 E. Friedliinder und Sohn. 1895, 42 pp. 



characters of the forms is changed quite gradu- 

 ally in a definite direction, determined by the 

 external conditions existing at that moment. 

 This process is called transformation of species 

 (Pfeffer), or mutation (Waagen, Neumayr, also 

 Scott, W. B., On Variations and Mutations, 

 Am. Journ. Sc, 48, 1894, pp. 355-374). 



It has been entirely overlooked by Weis- 

 mann and many others that the process of 

 mutation of one species or one group of forms 

 is by no means identical with the formation of 

 new, contemporaneous different species. Natural 

 selection can only operate in such a way as to 

 improve or modify, in the course of time, a 

 series of forms or a species, as soon as the con- 

 ditions of existence are changed; out of one 

 form another can be produced by selection, 

 but never two. The divergency of the direc- 

 tions of mutation, the origin of separate forms 

 from one ancestral form, natural selection 

 can never explain. This is only conceivable if 

 the conditions of existence are also differenti- 

 ated; that is, if they appear different in differ- 

 ent regions of the earth at the same time, so 

 that the ancestors of one form living in these 

 different locations are subjected to special con- 

 ditions. A successful effect of the different life 

 conditions, however, can only be imagined, if 

 the organisms are forced to remain permanently 

 in these conditions, if they are prevented from 

 migrating from one region of definite condi- 

 tions of existence into others with other condi- 

 tions. Therefore we have, as the fourth and 

 most important factor in the formation of dif- 

 ferent species, the separation in space or isolation. 

 I think everybody who has ever undertaken 

 to study the geographical distribution of certain 

 genera will agree with these views. I have 

 emphasized it frequently,* as is fully admitted 

 by Ortmann. The isolation prevents the cross- 



* Baur G. Das Variiren der Eidechsengattnng 

 Tropidurus auf den Galapagos Inseln. Biol. Cen- 

 tralbl. X. 1890, pp. 475-483. Leuckart. Festschrift 

 1892. pp. 259 fi. On the origin of the Galapagos Is- 

 lands. Am. Nat. 1891., pp. 217-229, pp. 307-319. 

 Ein Besuch der Galapages Inseln. Biol. Centralbl. 

 XII. 1892. pp. 221-250. The Differentiation of 

 Species on the Galapagos Islands and the origin of 

 the group. Biol. Lect. Mar. Biol. Lab. AVoods HoU 

 1894. Boston, 1895, pp. 67-78. 



