548 Causes and Course of Organic Evolution 



ferred to under Xemertinea (p. 420), Cyclostomata (p. 429), 

 etc., arise over various parts of the skin, that form electrotactic, 

 surface chemotactic, thermotactic, hydrotactic, mechanotactic, 

 and thigmotactic tissues. These all correspond functionally 

 to certain tissues in higher plants, if we except the tonotactic, 

 while in the animal, as in the plant, the response of the animal 

 when these tissues are stimulated is a positive or negative 

 chemotropic, heliotropic, paraheliotropic, thermotropic, geo- 

 tropic, electrotropic, mechanotropic, or hydrotropic one. 



The writer is unaware of any stimulation action in animals 

 which differs from that of plants, except it be the tonotactic, 

 and this, as several writers have suggested, is probably only a 

 highly sensitized and condensed expression of mechanotactic 

 stimulus. The epidermal system then, or some direct deriva- 

 tive of it, is that which receives, conveys, condenses, links up, 

 and finally responds to, all stimuli in the animal kingdom. 

 But, as compared with the diffuse condition in plants, definite 

 groups or areas of cells are set apart to receive definite stimuli. 

 It should however be kept in view that the very limited irrito- 

 perceptive area for geotropism in plants, and equally some 

 heliotropic areas, are as condensed as many animal centers of 

 impression. Similarly, the irrito-contractile centers or regions 

 in leaves and leaflets of Dioncea, Drosera, Mimosa pudica, etc., 

 are highly sensitive to several types of energy. 



It follows from the above that the comparative size, weight, 

 complexity, and tissue resistance of the nervous system may 

 furnish a fair index to the comparative biological dignity of 

 any individual, genus, or group. To a large extent it is the 

 recognition of this relation that has caused J. Loeb to give us 

 the remarkable and suggestive work " Comparative Physiology 

 of the Brain and Comparative Psychology." But further evi- 

 dence will be adduced to show that with the appearance of the 

 nerve cell, the first recognizable beginning of organ formation 

 for cogitic energy starts, and that, as the nerve cells become 

 increasingly multiplied, linked up, and concentrated, in cor- 

 responding ratio does the animal show increased intelligence, 

 memory, thought, reason, and complex volitional acts generally. 



