XXVI THE MAIN GROUPS OF ANIMALS. 



but for which Aristotle had no common name. This group, then, in 

 which he included Sponges, Sea-anemones, Jelly-fishes, Holothurias, 

 and Star-fishes, stood at the bottom of his scale of animal life. Their 

 inferiority was shown by even their common sensibility being so scantily 

 developed as to become sometimes of doubtful presence, while in none 

 of them were there any organs whatsoever of higher sense.' None 

 of them, moreover, possessed organs for active locomotion. Such of 

 them as changed place at all did so ajt the mercy of the waves and 

 currents, floating passively about " like plants detached from the soil " ; 

 to which the rest were more or less permanently fixed. As to the re- 

 production of these animals Aristotle says but little. He supposed them 

 to be developed spontaneously,'^ and to be altogether without organs 

 of generation, a character which, with the absence of a vent and their 

 simple structure generally, approximated them closely to plants. 



From this intermediate group we pass insensibly to true animals, that 

 is, to organisms whose sensibility is in disputabl e. These form two ^eat 

 groups, those that haVjel- blood, and those whose nutritive fluid iS'not 

 true blood but something '^ analogous to it ; a division which coincides 

 with the modern one, introduced by Lamarck, into Vertebrata and Inver- 

 tebrata. To this division of animals, into those with blood and those 

 without, it is objected that the one group has but a negative character. 

 The objection is drawn from Aristotle's own quiver, and is equally fatal 

 to Lamarck's Invertebrata. Aristotle's division may, however, be so 

 expressed as to avoid this criticism. Animals whose nutritive fluid is 

 jred, and animals whose nutritive fluid is white or colourless. But to this 

 again it is objected that some worms have coloured blood, and it may 

 be added that there is a fish whose blood is colourless. Similarly we 

 might object to Lamarck's division that there are fishes whose chorda 

 dorsalis is never replaced by vertebrae. Nomenclature is after all to a 

 great extent a matter of simple convenience ; and, when a convenient 

 name has been found for an undoubtedly natural group, exceptional 

 cases to which it scarcely applies, though they require to be noted, yet 

 hardly suffice for its displacement. "Vertebrata " will probably be retained 

 in spite of the exception to its accuracy given above ; nor are we likely to 

 discard the familiar " Reptilia," though the Pterodactyle flew and though 

 the Turtles swim. I am by no means sure then that Aristotle would have 

 abandoned his group-names even had he known of the exceptions to 

 their accuracy. But did he know of them ? Of the fish with colourless 

 blood, it need scarcely be said, he was ignorant. But earth-worms can 

 hardly have escaped his notice. It is strange, however, that he never 

 makes definite mention of them. It may be, nay probably is, the case, 



1 D. P. iv. 5, 43, etc. ^ H. A. v, 15, 21. ' D. G. iv. i. 30 ; D. P. ii. 3, 12. 



