0. — GEOLOGY. oS 



history of certain crinoids, I pointed out that the seriation due to the 

 migration of the anal plates must be checked by the seriation due to the 

 elaboration of arm-structure, and so on. 



In applying these principles we are greatly helped by DoUo's thesis 

 of the Irreversibility of Evolution. It is not necessary to regard this as 

 an absolute Law, subject to no conceivable exception. It is a simple 

 statement of the facts as hitherto observed, and may be expressed 

 thus: 



1. In the course of race-history an organism never returns exactly 

 to its former state, even if placed in conditions of existence identical with 

 those through which it has previously passed. Thus, if through adap- 

 tation to a new mode of life (as from walking to climbing) a race loses 

 organs which were highly useful to it in the former state, then, if it ever 

 reverts to that former mode of life (as from climbing to walking), those 

 oi'gans never return, but other organs are modified to take their place. 



2. But (continues the Law), by virtue of the indestructibility of the 

 past, the organism always preserves some trace of the intermediate 

 stages. Thus, when a race reverts to its former state, there remain the 

 traces of those modifications which its organs underwent while it was 

 pursuing another mode of existence. 



The first statement imposes a veto on any speculations as to descent 

 that involve the reappearance of a vanished structure. It does not 

 interfere with the cases in which old age seems to repeat the characters 

 of youth, as in Ammonites, for here the old-age character may be 

 similar, but obviously is not the same. The second statement furnishes 

 a guide to the mode of life of the immediate ancestors, and is applicable 

 to living as well as to fossil forms. It is from such persistent adaptive 

 characters that some have inferred the arboreal nature of our own 

 ancestors, or even of the ancestors of all mammals. To take but a 

 single point. Dr. W. D. Matthew (1904, Avier. Natural, xxxviii. 

 813) finds traces of a fomier opposable thumb in several early Eocene 

 mammals, and features dependent on this in the same digit of nil 

 mammals where it is now fixed. 



The Study of Habitat. 



The natural history of marine invertebrata is of particular interest 

 to the geologist, but its study presents peculiar difficulties. The marine 

 zoologist has long recognised that his early efforts with trawl and dredge 

 threw little light on the depth in the sea frequented by his captures. The 

 surface floaters, the swimmers of the middle and lower depths, and the 

 crawlers on the bottom were confused in a single haul, and he has 

 therefore devised means for exploring each region separately. The 

 geologist, however, finds all these faunas mixed in a single deposit. 

 He may even find with them the winged creatures of the air, as in the 

 insect beds of Gurnet Bay, or the remains of estuarine and land animals. 



Such mixtures are generally found in rocks that seem to have been 

 deposited in quiet land-locked bays. Thus in a Silurian rock near 

 Visby, Gotland, have been found creatures of such diverse habitat as 

 a scorpion, a possibly estuarine Pterygotus, a large barnacle, and a 

 crinoid of the delicate form usually associated with clear deep water. 



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