596 DERIVATION OF EXISTING SPECIES. 



such as bear upon the conversion of species of former times into those of the exist- 

 ing vegetation. 



A change in the conditions of life has, according to a widely-spread view, been 

 the immediate cause of a change in the vegetation. The altered conditions of life 

 provoke new wants in the plant, and these new requirements have led to a trans- 

 formation of their organs. Stimulated by use, the organs in question become 

 enlarged and further developed; others, no longer of service, become smaller, 

 atrophy, and disappear. It is the cumulative result of these small and almost 

 imperceptible changes that in course of time becomes apparent. These structural 

 changes are transmitted to the progeny, and with an increasing tenacity, the 

 greater the number of generations which have been exposed to the altered condi- 

 tions. This, the theory of adaptations, has provoked wide discussion and criticism. 

 It is urged against it that, whether wild or cultivated plants be considered, it is 

 only isolated or a few individuals, never the whole of the members of a species, 

 which exhibit these variations and transmit them to their offspring. If these new 

 characters are immediately due to the soil or climate, then all the individuals of a 

 species, exposed to like conditions of growth (environment), should exhibit them 

 and hand them on to their offspring. The permanence of the influence — and to 

 this many naturalists and others attach great importance — is without significance 

 in this matter. When a change is called forth — be it by an altered source of 

 nourishment, by the influence of heat or cold, light or darkness, moisture or dry- 

 ness — it must become apparent upon the growing plant, since a change in the plant 

 stands to a change in the environment as effect to cause. If the cause ceases, so 

 also does the effect, equally after the lapse of a year or a hundred years. But a 

 much more potent criticism of the theory of adaptation is the result of a series 

 of experiments which were carried out for the solution of these questions. From 

 them we see that an altered environment calls forth certain changes in the plants 

 submitted to it, but that these are not transmitted to the oflfspring, are not heredi- 

 tary, and that the influences of soil and climate do not provoke a fundamental 

 change in the constitution of the protoplasm. Influences of this sort can induce a 

 diseased condition in a plant and can even kill it, but they cannot bring about a 

 change which can be transmitted to the next generation. Though soil and climate 

 play a most important part in the struggle of species and varieties for existence, 

 and though the environment has a great influence on the origin of varieties and on 

 the distribution and migration of plants — as the immediate stimulus to the origin 

 of new and transmissible characters, and thus to the modification of species, change 

 of environment is without significance. 



Another theory dealing with the origin and modification of species is that 

 known as the theory of ^jrogressive transforrtiation by inherent forces. According 

 to it, the impulse to change resides in the inherent tendency of all species to 

 perfect themselves. This theory transcends all experience and depends on premises 

 and draws conclusions essentially metaphysical in nature ; it deals only in part 

 with the results of scientific observation. It presupposes a creation of living 



