160 Animal Life and Intelligence. 



modified, the movement does not return to its pristine 

 condition. By assimilation, they continually increase to a 

 certain size and then divide, and thus perpetuate in the 

 undulatory movement of successive generations the im- 

 pressions or resultants due to the action of external 

 agencies on the individual plastidules. On this view, then, 

 the form and structure of the organism are due to the 

 special mode of vibration of the constituent plastidules. 

 This vibration is affected by external forces. The modified 

 vibration is transmitted to the plastidules by the germ, 

 which, therefore, produce a similarly modified organism. 

 As Mr. J. A. Thomson says, "In metaphorical language, 

 the molecules remember or persist in the rhythmic dance 

 which they have learned." 



Darwin's hypothesis was frankly and simply organic 

 the gemmules are little germs. This of Professor Haeckel 

 tries to go deeper, and to explain organic phenomena in 

 terms of molecular motion. Mr. Herbert Spencer long ago 

 suggested that, just as molecules are built up, through 

 polarity, into crystals, so physiological units are built up, 

 under the laws of organic growth, into definite and special 

 organic forms. Both views involve special units. With 

 Mr. Herbert Spencer, their " polarity " is the main feature ; 

 with Professor Haeckel, their " undulatory movements." 

 According to Mr. Spencer, " if the structure of an organism 

 is modified by modified function, it will impress some 

 corresponding modification on the structures and polarities 

 of its units." * According to Professor Haeckel, the vibra- 

 tions of the plastidules are permanently affected by external 

 forces. In either case, an explanation is sought in terms 

 of molecular science, or rather, perhaps, on molecular 

 analogies. So far good. Such " explanation," if hypo- 

 thetical, may be suggestive. It may well be that the pos- 

 sibilities of fruitful advance will be found on these lines. 



But though, as general theories, these suggestions may 

 be valuable, they do not help us much in the comprehen- 

 sion of our special point. To talk vaguely about " undula- 

 * Herbert Spencer, " Principles of Biology," vol. i. p. 256. 



