6 JOURNAL OF THE [January, 



appearance of voluntariness which is exceedingly striking in the 

 movements of the free, boat-shaped diatoms. When you have a 

 specimen of the latter under your microscopically-aided eye, you 

 will see it slide over the bottom of your artificial pond without 

 discernible machinery but with very appreciable force and con- 

 siderable speed. If it meets an obstruction which is not too 

 large, it pushes it aside, but, if too heavy, it halts a few seconds 

 and then reverses its invisible engine and changes its course. 



Upon seeing this operation for the first time, one inevitably 

 jumps to the conclusion that he is witnessing the exercise of 

 what, in the highest orders of creatures, we call volition. But 

 again we need to restrain ourselves. Already this microscopic 

 vessel, — perhaps not more than the two-hundredth of an inch 

 long, — has brought us a freight of problems as big as the world ! 

 What is this diatom ? Is it really a living thing ? What is the 

 criterion of vitality, — what is life ? If the diatom is an organ- 

 ism, to which kingdom of animate beings does it belong ? And is 

 there a fundamental and absolute difference between animals and 

 vegetables, and, if so, what is it ? When we have disposed of 

 these weighty questions it will be time enough to approach that 

 other great mystery, — the physical basis of mind, — as Alfred Binet 

 has attempted to do in his essay on " The Psychic Life of Micro- 

 organisms." 



Of all the struggles through which scientific progress has been 

 accomplished, none is more interesting than the never-ending 

 conflict of opinion which has raged around the definition of life. 

 There is nothing about which ignorant and thoughtless people 

 are more certain than about their ability to distinguish between 

 that which is alive and that which is dead. On the other hand, 

 amongst the learned and the wise this is one of the unsettled 

 matters which as yet hold out no promise of being set at rest. 

 To the uninformed self-motion is the conclusive evidence of vi- 

 tality; — that is to say, motion not having an evident cause exter- 

 nal to the body moved. " It moves of itself, therefore it is alive," 

 is the offhand expression of the popular judgment. 



But if visible motion ever furnishes a trustworthy test, it is 

 under very strict conditions and in a very limited domam. That 

 it supplies no line of demarcation between the organic and the 

 inorganic worlds was made known in 1S26 by Robert Brown's 



