378 THE EVOLUTION OF LIFE. 



great indirectness, are the organs of external relation ; 

 which, in the progress of organic forms, undergo various 

 metamorphoses. Some light, too, is thus thrown on 



certain irregularities in the order of development of organs. 

 If we contemplate those continuous actions and reactions 

 which tend ever to establish a balance between an organic 

 aggregate and its units ; we shall see that the effect which 

 the units composing any organ, produce on the organism as a ' 

 whole, will depend, partly on the permanence of such organ, 

 and partly on its proportional mass. The influence of any 

 force, is a product of its amount multiplied into the time during 

 which it has acted. Hence, a larger part of the aggregate 

 acting for a shorter time, will impress itself on the phy- 

 siological units, as much as a smaller part acting for a 

 longer time ; and may thus begin to show its influence in 

 the developmental changes, as soon as, or even earlier than, a 

 part that has existed for a greater period. Thus it becomes 

 comprehensible why, in certain Sntozoa which have im- 

 mensely-developed generative systems, the rudiments of the 

 generative systems are the first to become visible. And 

 thus are also explicable, anomalies such as those pointed 

 out by Prof. Agassiz — the appearance, in some cases, of 

 traits characterizing the species, at an earlier period of 

 development than traits characterizing the genus. 



§ 132. So that while the embryologic law enunciated by 

 Yon Baer, is in harmony with the hypothesis of evolution, 

 and is, indeed, a law which this hypothesis implies ; the 

 minor nonconformities to the law, are also interpretable by 

 this hypothesis. Parallelism between the courses of develop- 

 ment in species that had a common ancestry, is liable to be 

 variously modified in correspondence with the later ancestral 

 forms passed through after divergence of such species. The 

 substitution of a direct for an indirect process of formation, 

 which we have reason to believe will show itself, both in the 

 unfolding of the entire organism and in the unfolding of par- 



