EEVIEWS. 



in past and present forms of life, are undoubtedly the strongest arguments in 

 favour of Darwin's theory of progressive development by natural selection. 

 But as geology alone must be the sole soarce of knowledge for testing or 

 learning the elfects of great periods of time in the gradual transmutation of 

 species, so will our efforts be resultant of efficient proof only in proportion to 

 the perfection or imperfection of the geological record. Tliis record Darwin 

 justly says is defective. No doubt, it is ; no doubt there are great gaps in the 

 earth's past history of which no trace remains — and many, and far more nu- 

 merous gaps which scientific investigations have not yet filled up. Still, we 

 may hope to find, and by patience and research no doubt we ultimately sliall 

 mark out, the great points in the pictui'e around which the details may reliably be 

 filled in by correctly drawn inferences. If we tabulate the number of known 

 species of any particular class of animals or plants, we find the numbers in- 

 variably in the aggregate ranging liigher untH 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 all the forms living during those eras in which such 

 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 Siluiian jDcriod with 317, and close in 

 the recent with 16,000. It foUows, then, that it in the pre-Silurian age life 

 began on our planet with the same number of definite type-plans, such as the 

 globular, the radiate, soft -bodied, the vertebrate, &c., which we see so prominently 

 defined in the existing races ; and taking the mollusca, 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 miraculous 

 interference of the Deity, then we have as a result an ever increasing ratio of 

 miraculous inteferences, and we must also regard creative energy as sixteen 

 thousand times more active in our time than in the pre-Silurian 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 Hue of which is representative of hundreds — is so 

 representative of the natural radiation of life-forms by the splitting 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 natural 

 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 existant 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 multipKed species seems thus to be con- 

 tinuously increased by a continuous and rapid decrease of available terrestrial 

 space by the ever increasing 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 mollusca. 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 



