254 CHAPTERS ON EVOLUTION. 



living beings, without exception, illustrate the process in question. 

 This remark has been made in reference to the developments we have 

 already studied. For example, there is not, after all, such an immense 

 difference between the development of an insect and that of a fish 

 or, for that matter, between that of the frog and of man himself when 

 the facts of development are fairly faced and duly understood. No 

 animal or plant is suddenly transformed into the perfect likeness of 

 its parent. On the contrary, it has not merely to grow, but it has to 

 be formed from that which is formless ; to become organised by the 

 development of that which has no structure at all ; and to advance 

 along lines of development during which it successively assumes a 

 transient likeness to the forms of other and lower beings. Thus a 

 quadruped, whilst undergoing development within its parent's body, 

 in reality passes through as strange and startling a metamorphosis as 

 does a frog outside its parent's body, and external to its egg likewise. 

 A quadruped is really at first like a fish and reptile. So alike are the 

 young of all vertebrates in their early stages, that recognition of the 

 nature of any particular form may be an impossibility. " Metamor- 

 phosis " thus occurs in quadrupeds as in frogs ; in snails and oysters as 

 in insects. The great and prevailing difference simply exists in the 

 fact that the insect or frog leaves the egg in an' imperfectly developed 

 condition and at an early stage of its career, passing the remainder 

 of its development as an independent being. In the quadruped 

 or fish, or in the bird and reptile, the young animal does not 

 quit the parent body or egg at such an early period, but remains 

 within its primitive shelter to undergo its full development or at any 

 rate to emerge upon the world of active life tolerably well prepared 

 for the struggle of living and being. Even amongst the quadrupeds, 

 as in well-nigh every other group of animals, and as in the plant 

 world likewise, there may be great differences in the degree and 

 stage of perfection at which the young organism is ushered into active 

 or independent existence. No better instance of this could be found 

 than in the case of the kangaroos and their allies, in which, as lower 

 quadrupeds, internal development ceases at a very 

 early period compared with that at which higher 

 quadrupeds are born. The newly born young of a 

 kangaroo, which, when full grown, stands 6 or 7 

 FIG. 169. f ee (. high, measures about one inch in length at 



YOUNG JVANGAROO. - . *?i" s \ i -ii ! i i 



birth (Fig. 169), and resembles a little red worm 

 much more nearly than a kangaroo. At birth it is transferred to 

 the characteristic " pouch " of the mother, wherein for weeks it 

 is protected and nourished by the milk secretion. If we consider 

 the effects of growth on such an organism, we may well feel 

 assured that a " metamorphosis " of very complete kind must be 

 required to transform the imperfect and feeble being just described, 



