EVOLUTION 239 



turned to the organism itself to discover the directive factor in 

 the production of new forms. They beheve that variation is not 

 a random process but that in any given species, or succession 

 of individuals, the variations tend always to be of a certain 

 particular sort, characteristic of that species, and that the species 

 consequently undergoes progressive change in a definite direction. 

 The advocates of such a theory of Orthogenesis, or internally 

 directed evolution, believe that evolutionary change is due to the 

 unfolding of certain tendencies in the protoplasm of the plant 

 or animal, and is not forced upon the organism from without. 

 They recognize the importance of natural selection in eliminating 

 the radically unfit, but believe this agency quite unable to create 

 anything new or to produce the organic world as we know it 

 today. 



It must be admitted that as yet we do not fully understand the 

 manner in which evolution has taken place and the factors which 

 have been responsible for it. In the past there has been perhaps 

 too much unsupported speculation on the problem and too little 

 pursuit of facts. The present intensive experimental study 

 of heredity, physiology, cytology and morphogenesis will, it is 

 to be hoped, provide us with a fund of information wherewith we 

 may attack this central problem of biology. 



QUESTIONS FOR THOUGHT AND DISCUSSION 



605. Why is it that we do not regard new strains of corn, apples, and 

 similar plants as new species? 



606. AVhy does the record of plant and animal evolution given us 

 by fossils have such large gaps in it? 



607. What types of plants and animals would be most likely to be 

 preserved as fossils? 



608. Characters which are apparently of the least functional impor- 

 tance to the plant are often most constant throughout large plant 

 groups and therefore very valuable in plant classification. Explain. 



609. Most of the members of the Figwort family have four stamens 

 in the flower, but there is often the rudiment of a fifth. The families 

 most nearly related to the Figworts have five stamens. What can you 

 infer from these facts as to tlie evolution of the flower in the Figworts? 



610. The plant population of an isolated island or island group iu the 

 ocean (such as the Hawaiian Islands) is composed very largely of species 



