122 RELATIONS OF MAN WITH EXTERNAL NATURE. 



animals and plants is easy, when attention is limited to the highest 

 forms in each kingdom of organic nature ; for no one can confound a 

 tree with a quadruped. In general, the organs of animals are far 

 more complex and numerous, and more specially devoted to particular 

 purposes^ than is the case in even the highest plants, the organs of 

 which, as the leaves, are mere repetitions of the single leaf, or, as .the 

 sepals, petals, and even stamens, pistil, and parts of the fruit, are but 

 modifications of the foliaceous organs. So also the tissues of animals 

 are more numerous and more complex, and the animal functions more 

 varied, than those of plants. 



But at the lower end of the animal and vegetable series the borders 

 of the two kingdoms become, as it were, conterminous, and the difficulty 

 of placing in their proper category some of the lowest organized forms, 

 infusorial and algaceous, has perplexed both zoologists and botanists. 

 Besides this practical embarrassment in regard to mere classification, 

 there exists a real difficulty in determining, by means of sharp defini- 

 tions, the differential characters between animals and plants generally. 

 The following distinctions, however, are those usually drawn. 



First, animals generally possess sensation, consciousness, and voli- 

 tion, whilst plants are certainly destitute of volition and consciousness, 

 and also of true sensation ; but there are a few animals which have no 

 volition, as the Spongida, and some perhaps which have no sensation, 

 as the Gregarinida ; again, there are many plants, the movements of 

 which are to some extent adapted to certain purposes, though they are 

 not volitional, such as the climbing plants, the sunflowers and others, 

 which turn in obedience to light or other stimuli; and there are some, 

 such as the sensitive plant, and the fly-catching plant, which possess 

 a sort of excitability, suggestive of, though not attributable to, true 

 sensibility in their leaves. 



Secondly, motion, especially locomotion, is a great characteristic of 

 animals ; whilst plants, as a rule, are stationary. But there are cer- 

 tain animals which are fixed, such as the sessile Polyzoa, Cirrhopods, 

 Actinozoa, Infusoria, and Sponjrida; and there are plants, such as the 

 Centaurea and Berberis, which manifest a true contractility of tissue, 

 and specific motions in their stamens; other plants, such as the Chara 

 and Vallisneria, exhibit remarkable movements in the contents of their 

 cells; whilst many of the forms among the minute Algae manifest active 

 locomotion, such as the Volvox, Oscillatoria, Zygnema, and certain 

 Monads. Whether such movements in plants are due to other causes 

 than contractility is not known ; a proper contractile tissue is certainly 

 more markedly developed in animals, existing even in the lowest forms, 

 such as the Rhizopoda and Spongida. 



Thirdly, there are certain differences between animals and plants, 

 as regards their food, and its mode of preparation, before it is ab- 

 sorbed. Thus, animals being locomotive, usually seek their food, 

 whilst plants, being fixed, find their food at the spot which they inhabit. 

 But there are sessile or fixed animals, and, amongst the Algae, moving 

 plants. Animals usually feed periodically, or at certain intervals ; but 

 plants much more continuously. The food of animals is both solid and 

 fluid, and the solid portions require to be dissolved by a digestive pro- 



