REVIEWS, 
409 
in past and present forms of life, are undoubtedly the strongest arguments in 
favour of Darwin's theory of progressive development by natural selection. 
l?ut as geology aloue must be the sole source of knowledge for testing or 
learning the elfects of great periods of tinu; in tiie gradual transmutation of 
species, so will our eil'orts be resiJtant of eliicient proof only in proportion to 
the perfection or imperfection of the geological record. This record Darwin 
justly says /*• defective. No doubt, it is ; no doubt there are great gaps in the 
cartii's ])ast history of wliich no trace remains — and many, and far more nu- 
merous gaps which seicntilic investigations have not yet filled up. Stdl, we 
may hope f o find, and by patience and research no doubt we ultimately sliall 
mark out, the great points m the picture around wliich the details may reliably be 
filled in by correctly drawn inferences. If wc tabulate the number of known 
species of any particular class of anunals or plants, we find the numbers in- 
variably in the aggregate ranging higher until we attain a maximum in the 
present creation, notwithstanding there are occasional deficiencies of individual 
genera between certain geological formations which shows that we have not yet 
a perfect knowledge of aU the forms living during those eras in which sucli 
deficiencies occur. Such results, however, are important in their bearing on the 
doctrine of the natural development of species. Taking for example the totals 
of known moUusca, we commence in the Silurian period with 317, and close in 
the recent with 16,000. It follows, then, that il in the pre-Sdurian age life 
began on our planet with the same number of definite type-plans, such as the 
globiUar, the radiate, soft-bodied, the vertebrate, &e., which we see so prominently 
defined in the existing races ; and taking the moUusea, for example, of that 
pre-Silurian period at unity, or as the first commencement of their special or 
direct creation; and regarding such special or direct creations as the mu-aculous 
interference of the Deity, then we have as a result an ever increasing ratio of 
mii-acnlous inteferenees, and we must also regard creative energy as sixteen 
thousand times more active in our time than in the pre-SUurian period. A 
condition of things few of us would be inclined to admit. On the other hand, 
this natural radiation of numbers — let us put it down by a diverging figure 
( see woodcut, p. 470) each Inie of which is representative of hundreds — is so 
representative of the natural radiation of life-forms by the spUttkig up of 
species by natural variations into new species, — one species first naturally 
divided into two species, these two into four, these four- into eight, and so on, 
as would naturally result by the operation of natural laws carrying on gradually 
and incessantly the transmutation and subdivision of old into new species, — as to 
incline the mind at first sight to faith in the past operations of such natui'al 
laws in the production of the very numerous species now living around us. 
The proportions of the level lines indicating the horizons of the various 
periods which the tree of increase of species does not cover, represents, of 
course, also the successively available spaces for the geographical spread of the 
species existaut at those dates, for the earth's surface cannot exceed a definite 
limited area, the maximum of which may be considered to be represented by 
the border lines of the diagram. 
The struggle for existence by the multiplied species seems thus to be con- 
tinuously increased by a continuous and rapid decrease of available terrestrial 
space by the ever increasmg sub-division and restriction of the geographical 
area. 
It should also be borne in mind that the diagram shows only the increase of 
specific forms of one class of the animal kingdom — the moUusca. Taking these 
as having increased from unity to sixteen thousand ; and taking the increase of 
all the other classes — the Radiata, Crustacea and insects, the Vertebrates terres- 
trial, aerial, or aquatic, &c., — as equal only to this sum in the aggregate of their 
similar specific increase, we have for the animal kingdom an assumed total of 
