498 TEXTBOOK OF BOTANY 



Within a group of related organisms, such as the species in a genus, 

 the different ones may be arranged in series such that some particular 

 character appears in ascending or descending order of magnitude. This 

 fact does not imply that the different species necessarily evolved in this 

 order, but it does imply that intrinsic conditions limit evolution. 



Evolution of organisms is limited in direction. That is, it does not pro- 

 ceed in all conceivable directions. 



The same general trends of evolution occur in organisms living in 

 very diverse habitats — for instance, in water and on land. 



Similar trends of evolution occur in different taxonomic groups of 

 organisms. 



Divergent trends of evolution occur in organisms living in similar 

 environments. 



Evolution may continue in a definite direction in a race of organisms 

 until it results in processes and structures that are handicaps to the race 

 and bring about its extinction. That is, "adaptation" is not the end of 

 evolution. 



Evolution may result in the development of useful organs and in the 

 subsequent deterioration of such organs to vestigial structures. 



Species formation is increased by geographic isolation, and by other 

 means that eliminate cross-fertilization. 



Adaptation. In an earlier chapter attention was called to the desir- 

 ability of periodically considering the appropriateness of terms used in 

 biology. The term adaptation was in the list cited at that time. It is often 

 stated that plants, and also animals, may become "acclimated" or 

 "adapted" to new conditions. One even hears that they adapt, or accli- 

 mate, themselves to new conditions. If this latter statement is true, there 

 is no need to discuss the matter, for there is no evidence that plants can 

 plan and direct changes within themselves to meet new conditions. To 

 say that they become acclimated, or adapted, states a condition that can 

 be experimentally investigated. We may arrive at an answer as to how 

 they become adapted, if they do. 



It may be untrue, however, to say that a plant becomes adapted to a 

 new condition. When we say that a plant is adapted to a certain condi- 

 tion we are merely saying that it can live, grow, and reproduce in that 

 condition. If it cannot live there — is not adapted — and we place it there, 

 it dies. It neither adapts itself nor becomes adapted. To do so, it would 

 suddenly have to become another kind of plant by an abrupt change in 

 its heredity. 



