164 Causes and Course of Organic Evolution 



brates, a still more varied functional diversity may be seen, 

 as in the skin glands and those of the alimentary tract (chap. 

 15) of nemerteans, etc. 



Again epidermal coverings, such as hairs, cutin, and cork 

 layers in plants, are as exactly comparable in primary origin 

 as they often are in function to the hairs, scales, and plates of 

 animals. 



When we review physiological responses similar results are 

 reached. Thus enough exhaustive work has been done on 

 nearly every group of plants as to enable us to say that, from 

 such unicellular organisms as Chlamydomonas or Piloholus up 

 through all intermediate groups to the highest flowering plants, 

 there is a definite set of environal stimuli, and as definite a 

 response by each organism to these. For example, the com- 

 bined geotropic and apoheliotropic growth of the mycelium 

 of Piloholus is in contrast to the delicately heliotropic growth 

 of the sporangiophore. The apoheliotropic, chemotropic, and 

 hydrotropic mycelium of Polyporus igniarius is continuous 

 vnlh. the delicately geotropic spore tubes of it. But, in experi- 

 mental study alike of motile and of fixed plant parts amongst 

 the Caryophyta, we have no evidence for considering that any 

 of these show "trial and error" relations. Already much lower 

 in the scale of evolving formation of inorganic colloid substances 

 and in cells of the most primitive acaryophytes, such stimulation 

 responses have already been fully determined. The physiologi- 

 cal responses then that are seen in the simplest Caryophyta 

 are those also wliich characterize the highest plants. The 

 intensity and the localization of these may vary even in related 

 species; their presence is fundamental. 



It would be impossible in a work like the present to advance 

 more evidence on the comparative "stock-in-trade" of plants 

 and animals. Enough we consider has been adduced that 

 will enable us to discuss intelligently the position taken in 

 the beginning of this chapter. The present brief- review, 

 fortified by abundant additional evidence that all can collate, 

 warrants the writer in accepting a widely extended and closely 

 uniform series of hereditary characters that remain typical 



