—The Line of Descent of the Invertebrata: 281 
each of the divisions of the animal kingdom from its first appear- 
ance in the geological series, through each successive stage, until its 
extinction or its continuance in the organisms still existent, with the 
view of representing in one work the different steps in the course 
of its development. It is, in reality, an attempt to co-ordinate the 
facts of paleontology with the theory of descent in the same way 
as has been successfully done with those of embryology and com- 
parative anatomy. 
In an introductory chapter, the nature of the evidence to be 
derived from paleontology, and its application to the theory of 
descent, are treated at considerable length. In contrast to that 
afforded by the study of living organisms, in which every structural 
detail can be determined, only the hard parts of fossils have been 
preserved, and these often in scattered fragments, with their original 
condition modified in varying degrees by fossilization, so that satis- 
factory conclusions as to the characters of fossil organisms can only 
be made by those investigators who, in addition to a thorough 
knowledge of the anatomy and development of existing animals, 
have acquired experience in interpreting the relations of mere frag- 
ments, and these often masked and distorted. The difficulties are 
also increased by the fact of the important differences between 
existing and extinct forms of life, the greater in proportion as we go 
back in time; and as some of the oldest forms differ altogether from 
any now existing, their characters must always remain enigmatical. 
It cannot be said that we have any satisfactory knowledge of the 
beginnings of life from the facts of paleontology. In the oldest 
known fossiliferous rocks of the Cambrian period there is already a 
great variety of different forms of life; and if these really repre- 
sented the oldest types, it would be strong evidence against the 
theory of descent. We can only suppose that long before Cambrian 
times, organic life may have existed through immeasurable periods 
of time, and this supposition is supported by the presence of 
graphite, anthracite, bitumen, and limestones—substances which are 
regarded as having an organic derivation—in the Archean rocks. 
The form known as Eozoon, however, is of too doubtful a character 
to be accepted as a proof of organic life. 
Not only are the earliest types of organic life missing, but 
throughout the geological series there are undoubtedly great gaps 
in the succession. The number of species in the existing animal 
kingdom approaches to 300,000, whilst there are only from 70,000 
to 80,000 fossil species. This great preponderance of existing forms 
is largely owing to the number of insect species, which constitute 
three-fourths of the total, whilst they are relatively rare as fossils. 
Of organisms inhabiting water, there are more fossil species known 
than recent; but, on the other hand, the existing species of land- 
dwellers are more numerous than those preserved as fossil. As an 
indication of the vast army of forms which in all probability have 
been wholly obliterated from our knowledge, the author states that 
there is good evidence to show that the marine life of the Jurassic 
seas was as various and abundant as that now existing; that during 
