1887. | Mr J. EB. Marr, On Homotaais. 75 
subdivisions, characterised by the occurrence of some dominant 
species in each, which we may call a, b,c, ..., the subdivisions of 
the two deposits may and probably will closely correspond with 
each other, as regards fossil contents, and yet the forms of the 
group a may have originated in the area of A, whilst other forms 
existed in B, and by the time the forms of the group @ have migrated 
to the area of B, the forms of the group b may have come into 
existence in A, and so on, 
Admitting that Professor Huxley’s warning was one which 
geologists required, it seems to me that many writers have been so 
impressed by it, that they have practically come to look upon the 
value of palzontological evidence as a means of comparing deposits, 
with extreme suspicion, and I propose, therefore, in this communi- 
cation to inquire whether this suspicion is justifiable, and if so, to 
what extent. In so doing, I shall consider chiefly the evidence 
afforded by the graptolites, not that this seems to me to be essen- 
tially different from that yielded by an examination of any other 
group of organisms, but firstly, because our knowlege of these forms 
is, thanks chiefly to the researches of Prof. Lapworth, very con- 
siderable, secondly, because I have paid more attention to these 
than to other life-forms; and thirdly, because there is perhaps 
greater reluctance to abide by the decision of the paleontologist 
in the case of the older rocks in which this group occurs, than in 
that of more modern deposits. 
B. Consideration of the facts of distribution. In order to lay 
before you the importance of the results achieved of recent years 
by a study of the Rhabdophora or graptolites, I shall have occasion 
to refer largely to Prof. Lapworth’s very important paper “On the 
Geological Distribution of the Rhabdophora.”* In this paper he 
shews that the Lower Palzeozoic rocks contain a series of grapto- 
litic faunas, which are marked by the constant association of certain 
forms, with some dominant form. To prevent overcrowding of 
detail, I give a list of such dominant forms, in the order in which 
they always appear, indicating at the same time the areas in which 
they have been discovered. Beginning with the oldest forms, we 
have 
Dictyograptus in Britain, Belgium, Scandinavia, America. 
Bryograptus — Britain, Scandinavia. 
Phyllograptus — Britain, Scandinavia, America, Australia. 
Murchisoni-form 
Didymograptus — Britain, France, Portugal, Scandinavia. 
Coenograptus — Britain, America, Australia. 
Amphigraptus — Britain, America, 
* Ann. and Mag. Nat. Hist. Ser. vy. Vol. m1. 
