THE WONDERS OF LIFE 



walking, speaking, etc.), may be repeated the next 

 moment as an unconscious reflex action. Again, there 

 are many important mixed or instinctive movements, 

 the impulse to which comes partly from internal and 

 partly from external stimuli. To this class belong 

 especially the movements of growth. 



Ever.y natural body that grows increases its extent, 

 fills a larger part of space, and so causes certain move- 

 ments of its particles; this is equally true of inorganic 

 crystals and the living organism. But there are im- 

 portant differences between the growth in the two cases. 

 In the first place, crystals grow by the external ap- 

 position of fresh matter, while cells grow by the intus- 

 susception of fresh particles within the plasm {cf. chapter 

 X.). In the second case, in growth, which determines 

 the whole shape of the organism, two impprtant factors 

 always co-operate, the inner stimulus, which depends on 

 the specific chemical constitution of the species, and is 

 transmitted by heredity, and the external stimulus which 

 is due to the direct action of light, heat, gravity, and 

 other physical conditions of the environment, and is 

 determined by adaptation (phototaxis; thermotaxis, 

 geotropism, etc.). 



A peculiar property of many vital movements (but 

 by no means all) is the definite direction they exhibit; 

 these are generally called purposive movements. For 

 the teleologist they afford one of the chief and most 

 welcome proofs of the dualistic theory of the older and 

 the modern vitalism. Baer, especially, has laid stress 

 on the purposiveness of all vital movement. It has been 

 given a more precise expression recently by Reinke. 

 His "dominants" are "intelligent directive forces," es- 

 sentially difEerent from all forms of energy or natural 

 forces, and not subject to the law of substance. These 

 metaphysical "vital spirits" are much the same as the 

 immortal soul of dualistic psychology or the divine 



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