116 RELATIONS OF MAN WITH EXTERNAL NATURE. 



an insect or a crustacean, however they may be modified or fused 

 together ; and so also the antennae, the jaws, the large pincers, and 

 the locomotive or swimming feet of the lobster, are obviously homolo- 

 gous parts resulting from the modification of typical lateral append- 

 ages. Homologies are also apparent between like parts of animals 

 constructed upon totally different types ; but they are fewer in number 

 and less apparent, sometimes obscure or even doubtful; thus there can 

 be no doubt that the stomach, salivary glands and liver of the Mol- 

 lusca, and even similar parts of the still lower subkingdoms, are 

 homologous with the alimentary canal and its appended glands in the 

 Vertebrata ; so also the supra-oesophageal ganglion of the Mollusca and 

 of the Annulosa, with which the nerves of special sense are connected, 

 is probably homologous, on the one hand, with the sensory portion of 

 the cerebrum of the Vertebrata, and is certainly so, on the other, with 

 the single ganglion of the Molluscoida, and of the Annuloid Rotifera. 

 But with regard to the locomotive organs in animals belonging to dif- 

 ferent subkingdoms, the homologies are not evident, as, e. </., between 

 the legs of quadrupeds, the legs of insects, and the locomotive organs 

 of the Mollusca. Further, as already mentioned, though the heart of 

 the Mollusca, and the dorsal vessel of the Annulosa, are really the 

 centres of the circulating system in those animals, it is doubtful whe- 

 ther they are strictly homologous with the heart of the Vertebrata ; 

 for the circulating organs in these lower animals are by some regarded 

 as possibly homologous with the lymphatic system of the Vertebrata, 

 in a few of which animals lymphatic hearts are met with. 



Homology must not be confounded with another, but less important, 

 relation which often exists between the parts of animals, and which is 

 designated by the term analogy. Parts or organs sometimes perform 

 corresponding functions, and yet are not homologous structures ; they 

 are then said to be analogous parts. Thus the lungs of the Verte- 

 brata are analogous to the pulmonary sacs of certain snails, and to 

 the air-sacs of spiders ; but, from their positions and connections, are 

 evidently not homologous parts. So, too, the wings of the bird and 

 those of the insect, and, again, the gills of the fish and the gills of the 

 lobster, are analogous, but not homologous organs. 



Although the essential resemblances between man and the higher 

 animals, on the one hand, and the lowest animals, on the other, become 

 at length so obscure, that the homology between a vertebrate animal 

 and a protozoon is no longer recognizable, so far as special organs are 

 concerned, still a profound homology remains throughout the whole 

 animal series, viz., that of the tissues. Thus, in the lowest animals, a 

 simple contractile protoplasm is the homologue of the highly compli- 

 cated muscular tissue of the higher animals, and the nerve-cells of the 

 most minute ganglion represent those of the large cerebrum even of 

 man. Moreover, in this point of view, as already mentioned, the 

 simple gymnoplastic nucleated cell-like Gregarina may be regarded as 

 homologous with a nucleated gymnoplast from the tissues of the higher 

 animals or of man. 



Still further, comparative physiology recognizes homologies in vital 

 actions. Homologous organs and tissues perform homologous func- 



