74 Mr J. E. Marr, On Homotazis. _ [Feb. 14, 
(2) On Homotaxis. By J. EK. Marr, M.A. 
A. Introduction of the term. The difficulty of proving the 
contemporaneity of rocks by their included fossils was shewn by 
Herbert Spencer in an article upon “ Illogical Geology,” reprinted 
in his “ Essays.” Ata later date, Professor Huxley, in his Anni- 
versary Address to the Geological Society, in 1862 (reprinted in 
his “ Lay Sermons”), insisted on the danger of confusing corre- 
spondence in succession with correspondence in age, and proposed 
the term “Homotaxis” (similarity of order), to express the simi- 
larity of serial relation of the faunas of strata of different areas. 
He observes that “whether the hypothesis of single or of multiple 
specific centres be adopted, similarity of organic contents cannot 
possibly afford any proofs of the synchrony of the deposits which 
contain them; on the contrary, it is demonstrably compatible with 
the lapse of the most prodigious intervals of time, and with inter- 
position of vast changes in the organic and inorganic worlds, be- 
tween the epochs in which such deposits were formed.” 
Again, “there seems, then, no escape from the admission that 
neither physical geology nor palzontology, possesses any method 
by which the absolute synchronism of two strata can be demon- 
strated. All that geology can prove is local order of succession. 
It is mathematically certain that, in any given vertical linear 
succession of an undisturbed series of sedimentary deposits, the 
bed which lies lowest is the oldest. In any other vertical linear 
succession of the same series, of course, corresponding beds will 
ocvur in a similar order; but however great may be the probability, 
no man can say with absolute certainty that the beds in the two 
sections were synchronously deposited. For areas of moderate 
extent, it is doubtless true that no practical evil is likely to result 
from assuming the corresponding beds to be synchronous or strictly 
contemporaneous ; and there are multitudes of accessory circum- 
stances which may fully justify the assumption of such synchrony. 
But the moment the geologist has to deal with large areas, or with 
completely separated deposits, the mischief of confounding that 
‘homotaxis’ or ‘similarity of arrangement,’ which can be de- 
monstrated, with ‘synchrony’ or ‘identity of date, for which 
there is not a shadow of proof, under the one common term of 
‘contemporaneity’ becomes incalculable, and proves the constant 
source of gratuitous speculations.” 
The danger of identifying widely separated strata as actually 
contemporaneous is of course due to the certainty, that, whether 
originating in single or multiple specific centres, they must have 
migrated outward from these. Consequently, if we have two 
deposits A and 6 remote from one another, containing the same 
dominant forms of life, and each capable of being split up into 
