l86 GEOLOGICAL BIOLOGY. 



iological, is not explained by simply calling it development. 

 Development is the history of the steps by which these char- 

 acters are attained. It is the term by which we express the 

 law of this history; and so long as the idea of the immutabil- 

 ity of species prevailed there was supposed to be a particular 

 law of development for each species. This law of develop- 

 ment was alike for all the descendants of a common ancestor. 

 By the expression law of development is meant a regularity 

 in the order of changes or in the sequence of steps by which 

 the results seen in the mature individual are attained. Every 

 descendant of a common parentage was thought of as 

 passing through the same stages of growth in reaching its 

 maturity. 



No Analogy between the Origin and Development of an Immuta- 

 ble Species. — The origin of the species from this point of view 

 was explained, necessarily too, in some other way than by 

 natural development ; the reverent were satisfied with consid- 

 ering it a special act of the Creator; others preferred to ex- 

 plain it by the fortuitous concourse of atoms. Neither found 

 any explanation in the natural laws of either generation or 

 development. That there are different species and that new 

 species have arisen were accepted facts; but the idea that 

 different species could be explained by any laws noted in the 

 development of the individual was not maintained. It was 

 believed that the characters were specifically distinct for each 

 species, and that this difference was in itself original. 



Inorganic Properties and Organic Characters Compared. — The 

 case was somewhat analogous to our idea of two kinds of 

 mineral substances, as gold and iron ; as to seeking an ex- 

 planation for their origin, we do not attempt it : we either say 

 they were created so in the beginning, or they appeared 

 spontaneously concurrent with cooling of the solar system. 

 Their differences we conceive of as their intrinsic properties. 

 So with the idea of immutability of species logically there 

 was associated the other idea, that the characters both mor- 

 phological and physiological are essential properties or 

 qualities of the species, and it was no more to be expected 

 that one would ask why do birds have feathers and dogs have 

 hair, than why is gold yellow and iron gray. The sufficient 



