502 CAUSE OF DIFFERENTIATION. 



and power thereby arising ; and this, in reality, is the structure of the 

 hydra just alluded to. Another advance is made by the preparation of 

 new and complicated structures, fashioned out in the substance between 

 the inner and the outer wall, and in this manner arise the various mech- 

 anisms for respiration and reproduction. Such a state of things is pre- 

 sented by the Actinia. 



It will be found, when we describe the development of the higher ani- 

 Individuai and mals, that a parallelism is observed between the career of 

 race develop- Q^h individual and that of the series to which it belongs, 

 niais by differ- The evidence furnished by natural history and paleontology 

 entiation. proves that, in the development of animal species, there has 

 been an orderly progress, not so much from those of a lower to those of 

 a higher form, as from the general to the special ; a gradual parting out 

 of structures and functions that were once commingled and coalesced, an 

 elaboration which may be attributed either to a melioration of the cir- 

 cumstances under which species were successively forming, or to the 

 innate power possessed by the organic structure itself. Even at the pres- 

 ent time our knowledge of the order of geological change is sufficiently 

 exact to enable us to institute an inquiry into the probability of the cor- 

 Differentiation rectness of each of these hypotheses, upon the principle that, 

 d h P 8?crfdr smce there is that parallelism between the career of individ- 

 cumstances. ual development and race development, there should also be 

 an analogy in the physical circumstances under which they have taken 

 place. Among conditions in animal development, two prominent ones 

 may be mentioned ; they are the degree of temperature at which the pro- 

 cess is carried forward, and the quality or nature of the medium supplied 

 for respiration. No doubt can now exist that, as regards the former, 

 there has been a gradual diminution from the early times, and that, as 

 respects the latter, the quantity of oxygen furnished in the medium of 

 respiration has been increased. It has long been observed, in a general 

 way, that there is a correspondence between the activity of respiration 

 and the degree of animal endowment, both as regards the individual and 

 Influence of ^ e race> The provision made for the more perfect conduc- 

 the aerial tion of the process from the moment that the embryo exhibits 

 any arterialization of its blood, is always attended with, if it 

 is not the cause of, increasing animal power. The supply of oxygen at 

 the first period is very imperfect, but instrumental means are introduced 

 in succession to increase the amount. When a mere membrane has be- 

 come insufficient to meet the requirements, branchiaB are resorted to, and 

 these, in their turn, are replaced by lungs. In a' double way, therefore, 

 an increased supply is secured, by alterations in the mechanism obtain- 

 ing it, which gradually becomes more and more adapted to the end in 

 view, or by variations in the chemical constitution of the medium which 



