xiv THE ANIMAL KINGDOM. 



or uses. Early in life other organs arise as outgrowths from the digestive tract of the 

 embrj'o. These are the lungs, the liver, pancreas, spleen, etc. 



There are also organs of suppoi-t ; such are the skeletons of animals, whether exter- 

 nal, as in the sea-urchin, starfish, lobster, or insect ; or internal, as in a fisli, bird, or 

 man. A true bony skeleton only exists in the vertebrates, or back-boned animals. 



TJie illustrious naturalist, Cuvier, established the principle of the correlation of 

 organs, showing that every organ must have close relations with the rest, and be more 

 or less dependent on the others. Each organ has its particular value in the animal 

 economy. There is a close relation between the forms of the hard and soft parts of 

 the body, together with the functions they perform, and the habits of the animal. 

 For example, in a cat, sharp teeth for eating flesh, sharp curved claws for seizing 

 smaller animals, and great muscular activity — all coexist with a stomach fitted for the 

 digestion of animal rather than vegetable food. So in the ox, broad grinding teeth 

 for chewing grass ; cloven, wide-spreading hoofs, that give a broad support in soft 

 ground, and a foui'-cliambered stomach, are correlated with the habits and instincts of 

 a ruminant. From the shape of a single tooth of an ox, deer, or a dog or cat, one can 

 determine not only its order, but its family or genus. Hence this prime law of com- 

 parative anatomy led to the establishment by Cuvier of the fundamental principles of 

 the science of palreontology, by which the comparative anatomist can with some degree 

 of confidence restore from isolated teeth or bones the probable foi-m of the original pos- 

 sessor. Of com'se, the more perfect the skeleton and teeth, the more perfect the remains 

 of the crust of insects or the shells of extinct molluscs, the more i^erfect will be our 

 knowledge, and the less room will there be for error in restoring extinct animals. 



New organs often arise by changes in the form and uses of simpler, older ones, 

 and the new organs may be so much changed by the gradual modification of its func- 

 tions as to assume new uses. " This fact," says Gegenlmur, in the introduction to his 

 Elements of Comparative Anatomy, "is of considerable importance, for it helps to 

 explain the appearance of new organs, and obviates the difficulty raised by the doc- 

 trine of evolution, viz., that a new organ cannot at once appear with its function 

 completely developed ; that it therefore cannot serve the organism in its first stages 

 whilst it is gradually appearing ; and that consequently the cause for its development 

 can never come into operation. Every organ for which this objection has the apjiear- 

 ance of justice can be shown to have made its first appearance with a significance dif- 

 fering from its later function. Thus, for exani])le, the lungs of the Vertebrata did not 

 arise simply as a respiratory organ, but had a predecessor among fishes breathing by 

 gills, in the swim-bladder, which at first had no relation to resj)iration. Even where 

 the lungs first assume the functions of a respiratory organ (Dipnoi, and many Am- 

 phibia) they are not the sole organ of the kind, but share this function with the gills. 

 The organ is therefore here caught, as it were, in the stage of con\'ersion into a respi- 

 ratory organ, and connects the exclusively respiratory lungs with the swim-bladder, 

 which arose as an outgrowth of the enteric tube and was adapted to a hydrostatic 

 function." 



Organs may also become modified liy disuse until they lose their distinctive form 

 and become rudimentary. Striking exam])les are seen in the parasitic Crustacea and 

 insects. As a result of the modification of organs by use and disuse, organs which are 

 morphologically the same become very different in function, and also in their general 

 a]i]iearanco ; so that we may classify organs differently either by their morphology or 

 physiological uses. 



