128 PERIOD V. 



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some future time ? To say nothing of the danger of 

 putting forth any such prophecy, it involves all the 

 consequences of descent with modification. The 

 opponent of evolution may as well admit at once that 

 the gill-breathing was practised in time past. As an 

 example of the same kind taken from plants, we may 

 quote the trifoliate leaves of the furze-seedling, which, 

 though absent from the full-grown furze, are frequent 

 in the family (Leguminosae) to which it belongs. The 

 general similarity of vertebrate embryos, of insect- 

 embryos and of dicotyledonous seedlings, is also worthy 

 of note. We may suppose that early embryos, being 

 largely or wholly dependent on food supplied by the 

 parent, and perhaps protected by the parent as well, 

 escape the pressure of the struggle for existence, and 

 are often not urgently impelled to produce adaptations 

 of their own. In these circumstances it is intelligible 

 that features inherited from remote ancestors should 

 persist. If, however, early independence is demanded 

 by the conditions of life, the embryo may develop 

 temporary adaptations, wanting in the parent and in 

 embryos of allied groups. Larval adaptation is as 

 much a part of the economy of nature as the retention 

 of ancestral structures which have been lost by the 

 adult. 



3. Morphology. — Let us next consider the light which 

 the Origin of Species throws upon homologous parts. 

 No example will serve our purpose better than the very 

 familiar one of the fore-limbs of different vertebrates, 

 the arm and hand of man, the wing of the bat, the wing 

 of the bird, the pectoral fin of the fish, and the paddle of 

 the whale. These limbs, adapted for actions so diverse 

 as grasping, running, flying, and swimming, neverthe- 

 less exhibit a common plan, evident at a glance, except 



