342 CHAPTERS ON EVOLUTION. 



XV. 



THE EVIDENCE FROM DEGENERATION. 



IT cannot be gainsaid that a survey of the fields of life around 

 us impresses one with the idea that the general tendencies of 

 living nature gravitate towards progression and improvement, and 

 are modelled on lines which, as Von Baer long ago remarked, lead 

 from the general or simple towards the definite, special and complex. 

 This much is admitted on all hands, and the ordinary courses of life 

 substantiate the aphorism that progress from low grades and humble 

 ways is the law of the organic universe that hems us in on every side, 

 and of which, indeed, we ourselves form part. The growth of plant- 

 life, which runs concurrently with the changing seasons of the year, 

 impresses this fact upon us, and the history of animal development 

 but repeats the tale. In the passage from seed to seed-leaf, from 

 seed-leaf to stem and leaves, from simple leaves to flower, and from 

 flower to fruit, there is exhibited a natural progress in plant existence, 

 which testifies eloquently enough, by analogy at least, to the existence 

 of like tendencies in all other forms of life. Similarly, in the animal 

 hosts, progressive change is seen to convert that which is literally 

 at first " without form and void " into the definite structure of the 

 organism. A minute speck of protoplasm on the surface of the egg 

 a speck that is indistinguishable, in so far as its matter is con- 

 cerned, from the materies of the animalcule of the pool is the germ 

 of the bird of the future. Day by day the forces and powers of 

 development weave the protoplasm into cells, and the cells, in turn, into 

 bone and muscle, sinew and nerve, heart and brain. In due season 

 the form of the higher vertebrate is evolved, and progressive change is 

 once more illustrated before the waiting eyes of life-science. But the 

 full meaning of most of the problems which life-science presents to 

 view is hardly gained by a merely cursory inspection of what may be 

 called the normal side of things. The by-paths of development 

 more frequently, perhaps, than its beaten tracks reveal guiding clues 

 and traces of the manner in which the progress in question has come 

 to pass. So, also, the side avenues of biology open up new phases of, 

 it may be, the main question at issue, and may reveal, as in the 

 present instance, an interesting reverse to the aspects we at first 

 deem of sole and paramount importance. For example, a casual 

 study of the facts of animal development is well calculated to show 

 that life is not all progress, and that it includes retrogression as well 



