THE EVIDENCE FROM DEVELOPMENT. 241 



development and descent could have been as plainly seen from the 

 outside world, as the frog's descent is traceable before our eyes to-day. 

 Higher development and progressive tendencies invariably tend to 

 shorten and condense the early stages of growth. Hence the value of 

 such cases as those of the frogs and their neighbours, which, through 

 mode of life, habits, and other and unknown conditions, have re- 

 tained much of their original "way of life," and have revealed to us, 

 through a literal byeway of development, the original and primitive 

 phases of that of all other animal forms. 



The conditions which favour or retard such developments are often 

 obscure, or very frequently unknown. The presence of water or its 

 absence, for instance, would favour or retard the continuance of the 

 metamorphosis in the frog-class. We must also bear in mind that 

 geological changes the rising and sinking of land and the like, the 

 conversion of swamps and morasses into dry land and similar physical 

 changes are powerful factors in producing modifications of habit in 

 aquatic animals, and, through change of habit, of effecting variations 

 in structure and form. It is possible to prove the existence and opera- 

 tion of such changes from many points of view. Both from the zoo- 

 logical or biological side, and from that of geology itself, the importance 

 of such alterations of the earth's surface can be proved. This aspect 

 of the subject finds appropriate illustration in works devoted to the 

 facts of geographical distribution and their explanation ; whilst we 

 may not neglect to observe the strictly utilitarian points involved in 

 such abbreviated life-histories as those we have been discussing. It 

 has been noted that as we ascend in the scale of animal and plant life, 

 development becomes more and more condensed and abbreviated. 

 On a posteriori grounds we might argue that, from the fact of such 

 condensation accompanying higher life and progressive development, 

 some obvious advantage in the struggle for existence was thereby 

 gained. The nature of such advantage is not difficult to discover. 

 The more prolonged and exposed larval or early existence is, the 

 more likely are the young forms to succumb from loss of food, 

 change of surroundings, or from the attack of enemies and numerous 

 other conditions. On the contrary, with an abbreviated infancy, the 

 animal obtains a distinct " coign of vantage." There is less risk of 

 early death, and a greater prospect of an earlier and stronger 

 maturity. Thus the " selected races " are those which possess the 

 shorter and more condensed life-history, and these races, therefore, 

 come to the front in the universal struggle for existence which besets 

 and surrounds the living hosts to-day as of yore. As Sir John Lubbock 

 remarks, when speaking of the shortening of the insect's life-history: 

 "The compression and even disappearance of those embryonal 

 stages which are no longer adapted to the mode of life which do 

 not benefit the animal is a phenomenon not without a parallel in 



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