344 CHAPTERS ON EVOLUTION. 



that life's forces tend as a rule towards progress, but likewise exhibit 

 retrogression and degeneration. If a living being is found to begin its 

 history, as all animals and plants commence their existence, as a speck 

 of living jelly, comparable to the adult animalcule of the pool, it is a 

 fair and logical inference that the organisms in question have descended 

 from lowly beings, whose simplicity of structure is repeated in the 

 primitive nature of the germ. If, to quote another illustration, the 

 placid frog of to-day, after passing through its merely protoplasmic 

 stage, appears before us in the likeness of a gill-breathing fish (Fig. 241), 

 the assumption is plain and warrantable that thefr$3g race has descended 

 from some primitive fish stock, whose likeness is reproduced with 

 greater or less exactness in the tadpoles of the ditches. Or if, to cite 

 yet another example, man and his neighbour quadrupeds (Fig. 242), 

 birds, and reptiles, which never breathe by gills at any period of their 



CALF. RABBIT. 



FIG. 242. EMBRYO-VERTEBRATES. 



existence, are found in an early stage of development to possess " gill- 

 arches " (), such as we naturally expect to see, and such as we find in 

 the fishes themselves, the deduction that these higher animals are de- 

 scended from gill-bearing or aquatic ancestors seems to admit of no 

 reasonable denial. On any other theory, the existence of gill-arches 

 in the young of an animal which never possesses gills is to be viewed 

 as an inexplicable freak of nature a dictum which, it is needless 

 to remark, belongs to an era one might well term prescientific, in 

 comparison with the " sweetness and light " of these latter days. 



Hanging very closely on the aphorism respecting development 

 and its meaning, is another biological axiom, well-nigh as important 

 as the former. If development teaches that life has been and still is 

 progressive in its ways, and that the simpler stages in an animal's 

 history represent the conditions of its earliest ancestors, it is a no 

 less stable proposition that at all stages of their growth living beings 

 are subject to the action of outward and inward forces. Every living 

 organism lives under the sway and dominance of forces acting upon 

 it from without, and which it is enabled to modify and to utilise by 

 its own inherent capabilities of action. It is, in fact, the old problem 



