xi THE ELEMENTS OF STRUCTURE 207 



less glands, like the thyroid gland and the adrenal body, 

 and are distributed through the body by the blood, often 

 affecting distant parts in a very specific way. From the 

 anatomical or morphological point of view, however, we 

 mean by the correlation of organs more especially the 

 fact that certain structures have been linked together in 

 the course of evolution. Thus no mammal chews the 

 cud that is not an even-toed Ungulate ; no animal that 

 has an allantois has gills. 



7. Homology, Analogy, and Convergence. When organs 

 either in the same or in different animals have a 

 similar development, and are built up on the same 

 architectural plan, they are called homologous. Those 

 whose resemblance is merely that they have similar 

 functions are termed analogous. Even Aristotle recog- 

 nised that some structures apparently different were 

 fundamentally the same, and no small part of the pro- 

 gress of morphology has consisted in the recognition of 

 homologies. Thus it was a great step when Goethe and 

 others showed that the sepals, petals, stamens, and 

 carpels of a flower were really transformed leaves, or 

 when Savigny discerned that the three pairs of jaws 

 beside an insect's mouth were really transformed legs. 

 To Owen the precision of our conceptions in regard to 

 homologies is in great part due, though subsequent 

 studies in development have added welcome corrobora- 

 tion to many of the comparisons which formerly w r ere 

 based solely on the results of anatomy. Thus an organ 

 derived from the outer embryonic layer (or ectoderm) 

 cannot be homologous with one derived from the inner 

 layer (or endoderm). Homologous organs in one animal 

 are well illustrated by the nineteen pairs of appendages 

 borne by a crayfish or lobster. These differ greatly in 

 form and in function ; many of them are not analogous 

 with their neighbours, one feels and another bites, one 

 seizes and another swims, but they are all homologous. 

 So are the different forms of fore-limb, the pectoral fin 

 of a fish, the fore-leg of a frog or lizard, the wing of a 

 bird, the flipper of a whale, the fore-leg of a tiger, the 

 arm of man. But the wing of an insect is merely ana- 



