104 NATURE 
this reservation ; the first forms were the simplest. It 
may be that as yet we know really very little of the dawn 
of geological history; that the fossiliferous rocks are 
nowhere near the real base. This conclusion has been 
derived by Prof. Poulton! from the complexity of the 
forms met with in them; still we find that we have not 
to deal with such a vast promiscuous association of plants 
and animals of lowest and highest organisation as we 
know to-day ; we deal relatively only with the simplest. 
The story both with regard to plants and animals is alike 
in this respect. 
Let me deal with the plants first. The first were 
aquatic—that is to say, they lived in and on the waters. 
So far as we know, the first plant life was akin to that 
of the algz which include our modern seaweed, moss- 
like plants followed them, and then ferns, and it is only 
very much later that the forms we know as seed plants 
with gaily coloured flowers living on the land made their 
appearance. The general trend of change amongst the 
plants has been in the direction of a land vegetation as 
opposed to one merely in or on the surface of the waters, 
and some present seaweeds exhibit the initial simplicity 
of plant-structure which characterised the beginning of 
vegetable life, while the seed plants I have mentioned are 
of comparatively late development ; but we still have our 
seaweed ; so that with all the change in some directions 
some forms like the earlier survive. 
After this explanation, relating to work in an apparently 
different direction, you will have no difficulty in under- 
standing the meaning I attach to the word “ evolution ” 
in relation to stars and the chemical elements which 
visibly exist in them, so far as the history of plant change 
is concerned ; but we are not limited to plant life. The 
same conceptions apply to animal life, and it is important 
for my subject that I should refer to that also. What do 
we find there? We are brought face to face with the 
same progression from simple to complex forms. This 
is best studied by a reference to the geological record 
Stratigraphical geology is neither more nor less than 
the anatomy of the earth,” and the history of the suc- 
cession of the formations is the history of a succession of 
such anatomies ; or corresponds with development as dis- 
tinct from generation. In stratigraphical geology, as can 
be gathered from any book on geology, we find the names 
of certain beds which contain certain different forms of 
animal and vegetable life. We begin with the Laurentian 
and Algonkian and then pass to the Cambrian, then to the 
Ordovician, the Silurian and Devonian, and so on through 
a long list of beds and geological strata until we come 
eventually to the Recent, that is to say, the condition of 
things which is going on nowadays on the surface of the 
earth. And if we prefer to map those many different beds 
into more generic groupings, we begin with the Primary 
or Paleozoic, we pass on to the Secondary or Mesozoic, 
and then we finally reach the Tertiary or Cainozoic. The 
deposition of these beds and of the animal life which has 
been going on continuously on the surface while those 
beds have been deposited gives us the various changes 
and developments which have taken place with regard to 
animal forms. It is worth while to go a little into details 
and to indicate the changes in these forms which have 
taken place, in the most general way. Beginning with 
the Lower Cambrian, we find that the animal forms were 
represented by Invertebrata such as Sponges, Corals, 
Echinoderms, Brachiopods, Mollusca, Crustacea and 
many early Trilobites ; not to mention true Fucoids and 
other lowly plant-remains. When we come tothe Silurian, 
we find a large accession of the above forms, especially 
of Corals, Crinoids, and Giant Crustaceans (such as 
Pterygotus) and armoured animals (Ostracodermi) with- 
out a lower jaw, or paired fins ; the beginnings of Verte- 
1 Presidential Address, Section D, British Association Meeting at Liver- 
pool, 1896. 
2 Huxley, Q. J. G. S., xxv. p. xliii. 
NO. 1544, VOL. 60] 
[JUNE 1, 1899 
brate life, not yet fully evolved, and one lowly organised 
group of armoured fishes named Cyathaspzs (without 
bone cells in their shelly-shield). Here, too, we meet 
with the first Air-breathers ; the wing of a Cockroach, 
and several entire and undoubted Scorpions! Thus in 
addition we get vertebrates as opposed to invertebrates, 
and the first traces of the fishes. In the advance to the 
Devonian the fishes (associated with giant Crustacea) pre- 
dominate; it has been called the age of fishes. In the 
next series, the Carboniferous, we find the first certain 
traces of amphibians, of which the early existence is like 
that of a fish: a state of things illustrated by the frog, 
which the majority of us in our early days have, 1 am 
sure, studied as a tadpole inits early stages ; and some of 
these amphibians still retain fish-like characters. It is 
not until we arrive at the Permian that the true reptiles 
are met with, but in the next great series, the Triassic, 
we meet with a remarkable evolutionary group of 
Reptiles, the Theriodontia, or beast-toothed animals, 
because (unique among reptiles) they possess a dentition 
like a dog or a lion, with incisors, canines and cheek-teeth ; 
the precursors doubtless of the succeeding Mammalian 
type. We pass easily thus from the reptiles to mammals 
which are related to them; for instance, the ornitho- 
rhynchus and the echidna are both Australian mammals 
which bring forth their young within the egg as do the 
reptiles. Well, after that we begin to deal with birds. 
The early birds were strikingly reptilian in some of their 
characters ; and the pterodactyle, which many of you 
may have seen remains of in different museums, was really 
a winged reptile and not a bird. From that we gather 
that mammals and birds are variants of reptiles. When 
we progress from the Jurassic to the Recent, we find man 
making his appearance as a direct descendant of all those 
early forms. ‘There is not much new in this. This, as I 
have said, is what Huxley largely demonstrated in this 
theatre thirty-nine years ago ina previous course of these 
Lectures to Working Men. 
When we come to study the life-history of the various 
forms brought before us by the geological strata, we 
find it to vary considerably, a fact indicated by the 
presence or absence of the different genera in the various 
strata. We find that the trilobites, for instance, only 
appear in the very early geological formations ; there is 
no trace of them in the recent, but if we take the 
annelids we find that they are continuous from the 
earliest to the latest formations ; we still have our worms. 
Again we find that certain other organic forms made 
their appearance very low down in the time scale, forms 
which were not represented at all in the earlier Cambrian 
and Silurian, and that some of these are continuous to 
the present day. Let us take the story of the fishes. A 
great many fishes made their appearance at the Devonian 
stage, there were few in the Silurian; some of these 
stopped there, whereas others have been continued from 
the Devonian times to our own. ‘Take, for instance, the 
Australian mudfish Ceratodus; to judge from the teeth 
this fish might well have lived on unchanged from late 
Paleeozoic times until the present day! We see there is 
a tremendous variation of possible life-range, so to speak, 
with regard to these different forms, and the plant 
record, although necessarily more imperfect than the 
animal on account of the nature of the organism, tells 
the same story in its fragmentary evidence. In that way, 
then, the geologist has been able to bring before us the 
continuity of life in various forms from the most ancient 
geological strata to the most recent. The record may 
be incomplete, but is complete enough for my purpose. 
But that is not the only evidence of evolution to which 
I can refer. 
The teachings of embryology confirm the argument 
based upon the study of geology, and suggest that the 
life-history of the earth is reproduced in the life-history 
of individuals. The processes of organic growth or em- 
