VARIOUS THEORIES OF SPECIES-FORMING 111 



ists who have no such high esteem of the value of natural 

 selection. These believe, variously, that (a) to the selection 

 factors other auxiliary or helping ones are to be added, or (6) 

 that various other factors are equally potent in species-forming, 

 or (c) that these other factors are the more important ones, or 

 finally (d) that the selection factors are of no importance at all, 

 that is, have no reality. Before Darwin, the French naturalist 

 Lamarck had clearly enunciated an explaining theory of species 

 transformation, and there are to-day many naturalists who 

 believe that the Lamarckian explanation, or its fundamental 

 assumption, is true, or, at least, that it is based on the more 

 important and effective factors in evolution. These natural- 

 ists have been called Neo-Lamarckians. Some of these have 

 formulated theories of their own based on Lamarckian funda- 

 mentals, but developed in directions more or less obviously away 

 from characteristic Lamarckism. 



Still other fundamental causal factors than the Darwinian 

 ones of selection and the Lamarckian ones of accumulated effect 

 of use, disuse, and functional stimulation are assumed in certain 

 other theories of species change and general evolution. Nageli, 

 a botanist and natural philosopher, believed in a special inherent 

 vitalistic principle or force in living matter which tends to pro- 

 duce progressive differentiation and evolution. Von Kolliker, 

 Korschinsky, and de Vries believe that species-forming occurs 

 by definite sudden small (or larger) fixed changes or mutations, 

 so that for them a mutational or discontinuous variation is the 

 fundamental causal factor in species transformation. Numerous 

 paleontologists believe that variation follows determinate lines 

 in its occurrence, so that evolution is orthogenetic, with its lines 

 primarily fixed by determinate variation. 



We may then examine briefly some of the more important 

 special theories or groups of theories put forward by biologists 

 either as auxiliary and subordinate to the more generally 

 known Darwinian theory, or as alternative with or substitutes 

 for this theory. 



First to be mentioned should be the transmutation theory 

 of Lamarck. In its simplest expression it is, that every individ- 

 ual organism is, throughout its lifetime, reacting to environ- 

 mental stimuli and conditions in such ways as to change its 

 structure and its habits in greater or less degree from the 

 structure and habits of its parents and ancestors, this change 



