486 THE POPULAR SCIENCE MONTHLY. 



2. It deals only with variations useful to the organism in its 

 struggle for existence, and can exert no power in fixing the end- 

 less number of what, from present knowledge, we are obliged to 

 consider fortuitous characters. It can not perpetuate useless 

 organs ; nor those of a vestigiary or obsolescent character.* 



Even with these restrictions, the principle is far-reaching and 

 profoundly important ; but it quite fails to account for many of 

 the most interesting manifestations of life that are obviously not 

 necessary or life-preserving, of which many will occur to every 

 one, such as, among lower organisms, many superficial details of 

 structure ; or, as among higher organisms, odd habits and cus- 

 toms, playful instincts, ethical traits, etc. Its limitations must be 

 narrowed. in proportion as we come to understand the other laws 

 of modification and the causes of variation in masses. Let us 

 briefly consider some of these causes. 



We soon find that they admit of a certain amount of classifica- 

 tion, the minor divisions of which, as in all systems of classifica- 

 tion, more or less fully interlock or blend. They fall, however, 

 into two chief categories, viz., (1) external conditions or environ- 

 ment, which are, at bottom, physical, and (2) internal tendencies 

 or promptings, which are, at bottom, psychical. We shall also 

 realize more fully that there is good reason for the varying im- 

 portance which has been placed on natural selection because it 

 represents a broad principle, based on the outcome of both these 

 categories, but particularly of the latter. Its value is not a fixed 

 one, and must needs change with the increase of exact knowledge 

 of the other factors, and did in fact change in the mind of its 

 originator. We shall further find that there are laws of evolution 

 which permit of formulation and expression, and which have in- 

 fluenced or controlled the mode of variation, but which must not 

 be confounded with or included among the causes of the variation 

 proper, though here, again, the line between the two kinds of 

 factors is not always easily defined. 



The conditions of organic modification may, therefore, roughly 

 be classed as (A) external and (B) internal, and these may be 

 almost indefinitely subdivided. The former class includes (]) 

 physical and (2) chemical forces, and in a broad way may be said 

 to induce modification independently of natural selection, how- 

 ever much this may act with them as a secondary cause. Certain 

 prominent features of the physical forces are worthy of mention : 

 as light, temperature, water (stagnant, or in motion), climate (un- 

 der which term may be included meteorologic phenomena, as elec- 

 tricity, atmospheric pressure, etc.), mechanics (gravitation, wind, 

 stress, friction, etc.) and geographies (migration, isolation, etc.), 



■""■ In the literature of evolution, those are usually termed rudimentary, but, strictly 

 speaking, this term should be applied only to nascent or incipient structures. 



