xxxii THE MAIN GROUPS OF ANIMALS. 



and in each the embryo was provided not only with an umbilical vesicle, 

 but also with an allantois.^ In all these characters Birds and Reptiles 

 resembled each other, and differed from Fishes.'^ The two groups then 

 were on a par in all their main characters. Still.we may, I think, fairly 

 assupie that the greater viy^ city of Birds, theiiAapid locomotion, their 

 mori^ varjed life, and theigcompar ative ly erect position, as contrasted 

 with the grovelling attitude of Reptiles, cannot but have led Aristotle 

 to assign to the former the place of honour. And this accords with 

 a passage' where he classes animals by the degrees in which the 

 parental instinct is developed. The purpose of Nature, he says, is to 

 give such an instinct to all animals. But in the lowest kinds the 

 instinct ceases to act with the deposition of the eggs. This applies to 

 the Bloodless animals, and as a rule to Fishes. In the next group the 

 instinct lasts till the eggs are hatched, and then ceases. This applies 

 to the Reptiles, whom he describes as laying their eggs on the ground 

 to be hatched by the heat of the soil, and coming backwards and 

 forwards at intervals to see that no mischief happens to them. A step 

 farther and we have animals of higher intelligence, that continue their 

 care until their young have not only been hatched, but have attained 

 full growth. Such are the Birds ; and most Mammals. While lastly 

 come man and some few animals of high intelligence, who retain their 

 affection for their young throughout life. 



Such were the main groups recognised by Aristotle, and such the 

 order in which he apparently placed them. It would, I think, not be 

 impossible, nor even very difficult, to apply the same tests to his minor 

 groups, and arrange these also pretty much in the order in which he 

 would himself have placed them. The zoophytes, for instance, would 

 have been classified by him, it may almost be said were * classified by him, 

 according to their apparent degrees of sensibility, their freedom from 

 attachment, and generally their less or greater resemblance to plants ; 

 the fixed sponges with their doubtful signs of feeling being at the 

 bottom, and the free and more manifestly sensitive star-fishes at the top, 

 while between them came groups less freely motor than the latter, and 

 yet more clearly sensitive than the former, which were " in fact virtually 

 plants, and nothing more"; namely the Holothuriae and Sealungs, which, 

 though not actually fixed, yet floated at the mercy of the waves like 

 "detached plants," and were as insensible as sponges, and the Acalephae, 

 which, though usually attached to the rocks, could, some or all, detach 

 themselves at will. Of the Testacea, again, we can hardly be wrong in 

 supposing that Aristotle would have placed the motor, and occasionally 



' Cf. iv. 12, note 15. " D. G. iii. I, iii. 2 ; iii. 3. 



=» D. G. iii. 2, 14. * D. P. iv. 5. 



