1869.] DAWSON — IDEAS OF DERIVATION. 135 



gills, or that the Siredou has, under certain circumstances, the 

 capacity to have its period of reproduction arrested until it has 

 gone on a stage further in growth and has lost its gills. In any 

 case the same species — nay, the same individual — is capable of 

 existing in a state of maturity as a creature half fish find 

 half reptile in regard to its circulation, or in a more perfect 

 reptilian state in which it breathes solely by lungs. Further, we 

 may suppose conditions of the earth's surface in which there 

 would only be Siredons or only Amblysomas, and a change in 

 these conditions inducing the opposite state. Here we have 

 for the first time actual facts on which to base a theory of 

 development. These facts point to the operation of two causes — 

 first, the possible Retardation or Acceleration of development, 

 and secondly, the action of outward circumstances on the organism 

 capable of this retardation or acceleration. We here substitute 

 for the tendency to vary of Owen's theory, the ascertained fact of 

 reproductive retardation or acceleration, and for the struggle for 

 existence, the action of changed physical conditions, and for the 

 question as to the change of one species into another, the 

 change of the same species from one genus into another. 

 Further, instead of vague speculations as to possible changes of 

 allied animals, we are led to careful consideration of the embryonic 

 changes of the individual animal, and as to the differences that 

 would obtain were its development accelerated or retarded. We 

 can thus range animals in genetic series within which anatomical 

 characters would show change to be possible. I cannot follow 

 these series out into the elaborate lists tabulated by Mr. Cope, 

 but may proceed to notice the limitations which his views 

 put to the doctrine of derivation. It is obvious that, if this be 

 the real nature of derivation as a possible hypothesis, then 

 derivation must follow the same law with metamorphism and 

 embryonic development. Those animals which undergo a meta- 

 morphosis must be those most liable to such changes ; for example, 



ft Batrachian would be more likely to be so than a true reptile, 



eonsequently those lower forms of animals in which metamorphosis 

 is most decided would be those in which derivation would be 

 most active, and when they had attained to a condition in which 

 metamorphosis is of less amount, the tendency to change would 

 be diminished. When we compare this with the actual succession 

 of animals in geological time, we can see, as many Palaeontologists 

 have remarked, that order of succession in time and order of 



