THE MAIN GROUPS OF ANIMALS. 



XXV 



by a single differentia, but requires many.^ That is to say, its position | 

 in regard to other groups must be determined by a consideration not/ 

 of one but of all its characters, and by striking a balance between those 

 points in which it excels and those in which it shows inferiority. How 

 inadequate a single test may be, and into what confusion it may lead, 

 Aristotle points out in the De Generatione (ii. i, 15), selecting the 

 apparent degree in which the instruments of the motor soul are deve- 

 loped as an example. So also he admits that the test on which he 

 chiefly relies in that treatise in judging of the heat of animals,^ namely, 

 the condition in which the young are produced, is not a perfectly sure 

 one ; other conditions, such as moistur e.^ having some influence, and 

 even caid itself sometimes producing indirectly the same result as heat.*' 



But though Aristotle thus refused to accept any one test of excellence 

 as sufficient, yet it is clear that he held some of his tests to be much 

 more trustworthy than others. What was wanted was a test that should 

 gauge the soul. But the soul is incorporate in matter, and such is the 

 uncertain character of matter that the bodily organs do not always 

 correspond with perfect strictness to the soul within. This makes it 

 " impossible to classify by functions common to body and soul." ' As 

 we can only judge of the soul through the body, every one of the tests 

 has this failing. This is partly obviated by taking many tests in place 

 of one, and by selecting those which give most direct information about 

 the soul, that is to say, with the least implication of matter. 



Aristotle divides all terrestrial things into three great primary groups: 

 |^5)^i''^&s without a soul, i.e. the Inorganic kingdom lYp^things with\ > 

 a purely nutritive soul, i.e. the Vegetable kingdom ; /Tt/ things with a I 

 soul that is not only nutritive but sensitive, i.e. the^tnimal kingdom^ 

 These three groups, he says, are not separated from each other by deep- 

 cut lines of demarcation ; but Nature passes from one to the other so 

 gradually and imperceptibly that it is sometimes difficult to say under 

 which heading a given object should be classed.* What group it was 

 with such dim vestiges of life as to bridge over the interval which 

 divided plants from the inorganic world, Aristotle does not say. We 

 may, however, fairly suppose that he had in his mind the Lichens and 

 the Mosses, of which latter Lord Bacon spoke as interposed between 

 corruption and life.'' But as to the transition from Plants to Animals he 

 is more explicit ; placing between them the indiscriminate collection or 

 organisms, which in after-times were confused together as zoophytes, 



' D. P. i. 3, 14. « D. G. ii. I, 23. 3 D G. ii. I, 18. * D. G. i. 10, and i. 11, i. 



* D. P. i. 3, 12. « H. An. viii. 1,5; D. P. iv. 5, 42. 



' Nov. Organ, xxx. "Moss, which holds a place between putrescence and a plant." 

 And in another passage, "Moss, which is but a rudiment between putrefaction and 

 a herb." 



