Ixxvi REPORT — 1866. 



other member of it : can it be asserted that the assigned limits of such groups 

 have a definite line of demarcation ? 



The condition of the earth's surface or, at least, of largo portions of it, 

 has for long periods remained substantially the same ; this would involve a 

 greater degree of fixity in the organisms which have existed during such 

 periods of little change than in those which have come into being during 

 periods of more rapid transition ; for, though rejecting catastrophes as the 

 general modus aijendi of nature, I am far from saying that the march of 

 physical changes has been always perfectly uniform. 



There have been doubtless what may be termed secular seasons, and there 

 have been local changes of varying degrees of extent and permanence ; from 

 such causes organized beings would be more concentrated in certain direc- 

 tions than in others, the fixity of character being in the ratio of the fixity 

 of condition. This would throw natural forms into certain groups whicli 

 would be more prominent than others, like the colours of the rainbow, which 

 present certain predominant tints though they merge into each other by 

 insensible gradations. 



While the evidence seems daily becoming stronger in favour of a derivative 

 hypothesis as applied to the succession of organic beings, we are far removed 

 from anything like a sufiicient number of facts to show that, at all events 

 within the existing geological periods capable of being investigated, there 

 has been any groat progression from a simj^ler or more embryonic to a more 

 complex tyjjc. 



Prof. HiTxley, though inclined to the derivative hypothesis, shows, in the 

 concluding portion of his address to the Geological iSociety, 1862, a great 

 nunibcr of cases in which, though there is abundant evidence of variation, 

 there is none of progression. There are, however, several groups of Vertebrata 

 in which the endoskeleton of the older presents a less ossified condition 

 than that of the younger genera. lie cites the Devonian Ganoids, tlie Mcso- 

 zoic Lcpidostcidsc, the Pala?ozoic Sharks, and the more ancient Crocodilia 

 and Laccrtilia, and particularly the Pycnodonts and Labyrinthodonts, as 

 instances of this Avhcn compared Avith their more recent representatives. 



The records of life on the globe may have been destroyed by the fusion of 

 the rocks, which would otherwise have preserved them, or by crystallization 

 after hydrothermal action. The earlier forms may have existed at a period 

 when this planet Avas in course of formation, or being segregated or 

 detached from other worlds or systems. We have not evidence enough to 

 speculate on the subject, but by time and patience we may acquire it. 



Were all the forms which have existed embalmed in rock, the question 

 would he solved ; but what a small proportion of extinct forms is so preserved, 

 and must be, if we consider the cii'cumstanccs necessary to fossilize organic 

 remains. On the dry land, unwashed by rivers and seas, when an animal 

 or plant dies, it undergoes chemical decomposition which changes its form ; it 

 is consumed by insects, its skeleton is oxidized and crumbles into dust. Of 

 the myriads of animals and vegetables which annually perish, we find hardly 

 an instance of a relic so preserved as to be likely to become a permanent 

 fossil. So again in the deeper parts of the ocean, or of the larger lakes, the few 

 fish there are perish and their remains sink to the bottom, and are there fre- 

 quently consumed by other marine or lacustrine organisms or chemically de- 

 composed. As a general rule, it is only when the remains are silted iip by 

 marine, fluviatile or lacustrine sediments that the remains arc preserved. 

 Geology therefore might be expected to kecj) for us mainly such organic 

 remains as inhabited deltas or the margins of seas, lakes, or rivers; here 



