ox THE HYPOTHESIS OF EVOLUTION. I39 



things, the skeleton is represented by cartilage only, and the sala- 

 mander's is ossified. This relation is therefore an imitation only, 

 and is called inexact parallelism. 



As we compare nearer and nearer relations — i. e.^ the genera 

 which present fewest points of difference — we find the differences 

 between undeveloped stages of the higher and permanent con- 

 ditions of the lower to grow fewer and fewer, until we find numer- 

 ous instances where the lower genus is exactly the same as the 

 undeveloped stage of the higher. This relation is called that of 

 exact parallelism. 



It must now be remembered that the permanence of a charac- 

 ter is what gives it its value in defining genus, order, etc., in the 

 eyes of the systematist. So long as the condition is permanent no 

 transition can be seen ; there is therefore no development. If the 

 condition is transitional, it defines nothing, and nothing is devel- 

 oped ; at least, so says the anti-developmentalist. It is the old 

 story of the settler and the Indian : "Will you take owl and I 

 take turkey, or I take turkey and you owl ? " 



If we find a relation of exact parallelism to exist between two 

 sets of sj^ecies in the condition of a certain organ, and the differ- 

 ence so expressed is the only one which distinguishes them as sets 

 from each other — if that condition is always the same in each set 

 — we call them two genera : if in any species the condition is va- 

 riable at maturity, or sometimes the undeveloped condition of the 

 part is persistent and sometimes transitory, the sets characterized 

 by this difference must be united by the systematist, and the whole 

 is called a single genus. 



We know numerous cases where different individuals of the 

 same species present this relation of exact parallelism to each 

 other ; and, as we ascribe common origin to the individuals of a 

 species, we are assured that the condition of the inferior individual 

 is, in this case, simply one of repressed growth, or a failure to ful- 

 fill the course accomplished by the highest. Thus, certain species 

 of the salamandrine genus Amblystoma undergo a metamorphosis 

 involving several parts of the osseous and circulatory systems, etc., 

 while half grown ; others delay it till fully grown ; one or two 

 species remain indifferently unchanged or changed, and breed in 

 either condition, while another species breeds unchanged, and has 

 never been known to complete a metamorphosis. 



The nature of the relation of exact parallelism is thus explained 

 to be that of checked or advanced growth of individuals having a 



