676 LIFE : OUTLINES OF GENERAL BIOLOGY 



growths from the anterior region of the food-canal; they are lined 

 internally with endoderm and covered externally with mesoderm. 

 But when organs have not only the same general development but 

 the same fundamental structure, their resemblance is still closer, 

 and they are said to be homologous. Thus the fore-limb of a frog, the 

 fore-paddle of a turtle, the wing of a bird, the flipper of a whale, 

 the wing of a bat, the fore-leg of a horse, the arm of man, are all 

 homologous. They start in development in the same way as minute 

 lateral buds, and they show the same fundamental bones, muscles, 

 nerves, and blood-vessels. Homology is general resemblance in 

 development and in fundamental structure, entirely irrespective of 

 what the use of the part or organ may be. 



Different parts of the same animal may be homologous, and they 

 are termed serially homologous, when they are several times 

 repeated. Thus the lobster's nineteen pairs of appendages (unless 

 possibly the first two) are serially homologous, although they show 

 great diversity of function and of final form. It was a great step 

 long ago when Savigny showed that the three pairs of mouth-parts 

 or "jaws" are homologous with the insect's legs, being, in a word, 

 "appendicular". But the idea of homology is the same whether the 

 parts compared are in one animal or in different animals ; hence one 

 of the memorable diagrams in the history of zoology is that in which 

 Belon (1555) placed side by side the very different skeletons of bird 

 and man, and indicated the homologies of their various bones. 



Morphological Theory of the Flower. — Here we may refer 

 to one of the clarifying ideas in morphological botany — that the 

 flowering plant consists of an axis bearing various homologous 

 appendages of a leaf -life or foliar nature. The cotyledons or seed- 

 leaves, the radical and cauline leaves at the base and higher up the 

 stem, the bud-scales and the bracts, besides the sepals and petals, 

 the stamens and carpels, may all be regarded as homologous. It is 

 for the young student a never-to-be-forgotten lesson to make and 

 draw a series from the outer scales to the yoimg palmate leaves in 

 the opening bud of the horse-chestnut, or a series from green sepals, 

 through petals, to the stamens of the water-lily. The stamens and 

 carpels are no doubt the bearers of spore-forming organs (anthers 

 and ovules), but in themselves they are foliar, as Goethe said. This 

 is indicated not only by their mode of development, and by the 

 occurrence of transitions between petals and stamens, but also by 

 the resumption of more leaf-like features when the flower relapses 

 into vegetativeness and becomes "double", the stamens being in 

 many cases replaced by petaloid structures. 



The idea of the fundamental identity of floral parts and leaves is 

 an old one, and was clearly expressed by Linnaus in the aphorism 

 Principium florutn ei foUorum idem est. Even earlier there were 



