210 BOTANY 



and the Cycadeae ; they have leaves like the former but seeds like 

 the latter, while anatomically they present resemblances to both. 



5. DIRECT EVIDENCE OF THE VARIABILITY OF SPECIES. All the 

 preceding sources of evidence gain in significance from the direct 

 observation of the inconstancy of some species. Careful observation 

 establishes the appearance, both under natural conditions and, more 

 frequently, in cultivation, of inheritable deviations which would have 

 the systematic rank of varieties or species. It has also been possible 

 in various ways to experimentally produce new forms the characters 

 of which are inherited. The importance of such observations is that 

 they give some insight into the problem of the formation of species 

 and the origin of new morphological characters. 



B. Formation of Species and the Origin of Adaptations. All 

 observations have so far shown that the inheritable changes in 

 organisms may concern this or that character, may be larger or smaller, 

 and are irregular in origin. This serves to elucidate the great variety 

 in organic forms. These abrupt changes may be harmful, indifferent, 

 or useful to the organism. If they are so injurious that the life of 

 the organism is scarcely possible, the variety will disappear as quickly 

 as it originates (e.g. seedlings that have lost the power of forming 

 chlorophyll). To what extent such inheritable changes arise under 

 the influence of external conditions has yet to be determined in 

 particular cases ; it will be treated of in the physiological portion of 

 this text-book. 



Since the acceptance of a theory of evolution it has been evident 

 that the origin of the ADAPTIVE CHARACTERS of organisms called for 

 special explanation. The recognition that living beings vary in all 

 directions does not afford insight into the striking fact that organisms 

 are in many ways adapted to their environment, and organs more or 

 less adapted to their functions, while the reactions of the organisms are 

 beneficial. This condition of adaptation or inherited adaptedness 

 must in some way have originated phylogenetically. As to how it 

 arose, observations and experiments have to the present given no 

 direct answer. Explanations have been sought in a different way, 

 the two most important hypotheses being known as Lamarckism and 

 Darwinism. 



1. Lamarekism ( 105 ). This hypothesis starts from the fact that 

 some organisms assume a different form according to the surroundings 

 in which their germ cells develop to the mature organism, without 

 losing the power of developing differently in another environment. 

 Thus there are plants which can live both on land and in the water 

 (amphibious), assuming different forms according to the environment. 

 When grown on land they have the form and internal structure of 

 typical land plants ; when cultivated in water they resemble typical 

 aquatic plants. Some plants under dry conditions of cultivation 

 produce xerophilous characters, while when grown in moist air they 



