VARIOUS THEORIES OF SPECIES-FORMING 113 



sure ; ascent is not up a single ladder or along a single genealogi- 

 cal branch, but these branches are few (as indeed we actually 

 know them to be, however the restriction may be brought about) 

 and the evolution is always progressive, that is, toward what wo, 

 from an anthropocentric point of view, are constrained to call 

 higher or more ideal life stages and conditions. 



Other naturalists also seeming to see this course of determin- 

 ate or orthogenetic evolution, but not inclined to surrender their 

 disbelief in vitalism, in forces over and beyond the familiar 

 ones of the physicochemical world, have tried to adduce a 

 definite causomechanical explanation of orthogenesis. The 

 best and most comprehensible types of this explanation are 

 those essentially Lamarckian in principle, in which the direct in- 

 fluence on living matter of environmental conditions, the direct 

 reactions of the life stuff to stimuli and influences from the 

 world outside, are the causal factors in such an explanation. 

 But while every naturalist will grant that such factors do change 

 and control in considerable degree the life of the individual, 

 most see no mechanism or means of extending this control 

 directly to the species. 



The stumbling block of heredity, the means and mode of 

 inheritance, as we so far know them, are directly in the way of 

 any general acceptance of such a theory of evolution under the 

 direct control of such "primary factors of life. ;; Ontogenetic 

 species, that is, conditions of structure and habit common to 

 many individuals of one kind, the conditions due to sameness 

 of intrinsic and extrinsic factors in development, constitute a 

 category of organisms which at any given time and place seem 

 very real, and are for the moment truly real. But their 

 environment is remaining fairly constant. We speak easily of 

 the flux of Nature: her everchangingness. And in the large 

 we are speaking only of the truth. But during our brief 

 period of observation of the few generations of this or that kind 

 of animal or plant that come under our eyes and microscopes, 

 the nature environing these generations may be nearly uniform. 

 What are the changes in the desert in a score or a hundred or a 

 thousand of years? What changes in life conditions on the 

 barren storm-swept peaks of the mountain ranges? What in 

 the waters of that brackish bay or sweet-water lake apart from 

 the paths of man? Ontogenetic species have a seeming of 

 reality, but so far as our present knowledge goes it is only a 



