l36 THE CANADIAN NATURALIST. [June 



embryonic development correspond with one another to a 

 remarkable degree. We see also, however, that in the higher 

 animals changes of species have taken place more rapidly than in 

 those of lower grade, though in the latter metamorphosis is 

 usually more marked — a fact not apparently in accordance with 

 our hypothesis. 



According to this view, also, a species once created may have 

 in itself a capacity for passing through several generic forms, 

 constituting a cycle which ever tends to return into itself, or 

 to advance and recede by steps more or less abrupt under the 

 law of retardation and acceleration, combined with the influence 

 of external circumstances. Yet the dimensions of the orbit of 

 each species must be limited, its duration in time must also be 

 limited, and its capacity to pass into a really new species must 

 still be a point subject to doubt, but open to anatomical 

 investigation and inference. As already hinted, it is a most 

 important point of this theory, that when we have ascertained 

 the series of embryonic changes of any animal, we have thereby 

 ascertained its possibilities in regard to accelerated develop- 

 ment. Its possibilities in regard to retarded development may 

 be inferred by similar studies of animals higher in the scale. 

 Now, if we knew the embryonic history of every animal recent 

 and fossil, in its anatomical details, we should be able to construct 

 out of this a table of possible affiliation of animals, and should be 

 able to trace our existing species through the same genera, 

 families, orders and classes in which they might have existed in 

 geological time, and to predict what they might become in time 

 still to come. This hypothetical scheme of creation would 

 approach to the actual one in as far as we were able to correlate it 

 with the physical changes which have occurred or will occur on 

 our planet. Let us take as a crucial test the case of man 

 himself. The actual anatomical and physiological differences 

 which obtain between those races in which maturity is latest, and 

 those in which it is earliest, and a comparison of these with 

 embryonic characters, would give us the modern data. The 

 comparison of these with the most ancient human remains might 

 enable us to infer whether retardation or acceleration has been 

 the tendency in historic or geological time. From this we might 

 infer what might be the condition of man under a still more 

 accelerated development than any now known, or under that 

 antediluvian condition in which immaturity is said to have been 



