66 General Part. 



te comparedj for example, with the Whale ; Seals, Otters ; or 

 other digging forms with the Mole; the Mole-cricket and Rabbits, 

 Dung-Beetles. 



Sessile Animals. — In spite of the fact that the power of free- 

 locomotion is specially characteristic of the Animal Kingdom, there 

 are yet many forms which are fixed to one spot for at least the 

 greater part of their lives. The fixation usually results from 

 a certain part of the surface of the animal coming into connection 

 inseparably with a foreign object, a stone, a dead Mollusc-shell, or 

 the surface of another animal; the connection is often effected by 

 a cuticular secretion. Frequently, as in many Ch^topods, the 

 connection is less intimate, for the shell only, with which the animal 

 is not in direct union, from which, it may escape, is fixed to the 

 foreign object. The transition from the free-living to the sessile 

 form, is given by those animals which, though capable of locomotion, 

 yet generally remain for some time (days or years) in the same 

 place (some Gastropods, Lamellibranchs, Fresh-water Polyps, and 

 others). 



The immediate and very natural result of a sessile life is the 

 atrophy of the locomotor organs. The mouth in sessile forms 

 (Corals, Hydroids, Tubiculous-worms, Polyzoa, Brachiopoda) is very 

 often surrounded with long, prehensile arms or tentacles, which 

 are adapted either for seizing those animals which happen to come 

 near, or, by means of their ciliated border, for driving small organ- 

 isms into it. In other sessile animals (Vorticella, Sponges, Oysters), 

 the^jtentacles are wanting, but small organic particles are conveyed 

 to the digestive organs by ciliary currents. This mode of life is 

 further obviously favourable to the formation of colonies: most 

 colonies are sessile, especially those which are arborescent. 



4. The Transforming Effects of the Environment. 



In the foregoing paragraphs, cases of adaptation to the 

 environment have been examined : some facts illustrating its direct 

 influence upon organisms will now be considered.* 



It is a matter of universal observation that organs, at least in 

 many cases, improve with use, whilst they degenerate with disuse. 

 This may be seen, on the one hand, in the great development of the 

 arm, in people employed in manual labour : on the other, in the 

 feebler condition of this part, in those whose work is entirely mental. 

 Similar facts are also known for domestic animals. 



It has further been proved in various instances that a special 

 food may have a modifying effect upon the colour of certain 

 birds. A reddish - yellow colour may be quickly produced in 



* There is stiU much to be done in this connection so that the following remarks 

 must be taken somewhat tentatively. 



