878 ECOLOGY 



natural selection seems to explain such structures quite as inadequately 

 as did the old and discredited theory of special creation, it is not possi- 

 ble as yet to put one which is adequate in its place. Perhaps the most 

 tenable theory is that of orthogenesis. This theory postulates a definite 

 trend in the course of evolution, regardless of the influence of selection. 

 It would assume that the specialized features of flowers and also of 

 insects are organization characters that are more or less inherent in the 

 species. According to this conception the insects and flowers are not 

 adapted to each other, but insects in their floral visits select those flowers 

 whose structures happen to be suited to their mouth parts. It is obvious 

 that this still leaves unanswered the most fundamental question of all, 

 namely, the cause of floral structures. In the present state of knowledge, 

 it is not possible to say whether the evolution of floral structures has been 

 determined chiefly by external factors or by factors that we call internal. 

 This subject, in so far as it has to do with external factors, belongs 

 properly to the following section. 



3. THE INFLUENCE OF EXTERNAL FACTORS 



UPON THE 

 DEVELOPMENT AND FORM OF REPRODUCTIVE ORGANS 



Introductory remarks. Variations in the development and form 

 of reproductive organs are less common than are similar variations in 

 vegetative organs, but they are much more common than has been sup- 

 posed. Their relative invariability has long made differences in repro- 

 ductive structures the chief basis of classification. For this very reason 

 variation is nowhere more significant, since, if the present theories of 

 classification are correct, the study of reproductive variations, however 

 few or inevident they prove to be, may lead to the interpretatic n of 

 evolution. The possibilities of experimentation in this field are well 

 shown by a recent study of the fungus, Saprolegnia; from a single my- 

 celium there have been derived by appropriate changes in the media 

 the forms of asexual reproduction that have been held to be charac- 

 teristic of six different genera. 



Reproductive variation in the seedless plants. Experimental data 

 from the algae and the fungi. In the seed plants it is common to speak 

 of two contrasting states, namely, the vegetative and the reproductive, 

 but in many algae there are three such states, characterized respectively 

 by vegetative activity, by asexual reproduction, and by sexual reproduc- 



