624 - PALAONTOLOGICAL GEOLOGY. [Boox V. 
been well understood that the stratified portion of the earth’s crust - 
contains a suite of organic remains in which a gradual, progression 
can be traced from simple forms of invertebrate life among the early 
formations to the most highly differentiated mammalia of the 
present time. Until the appearance of Darwin’s “ Origin of Species” 
in 1859 the significance of this progression and its connection with 
the biological relations of existing faunas and floras were only dimly 
perceived. Darwin, however, urged that, instead of being fixed or 
but slightly alterable forms, species might be derived from others, 
and that processes were at work whereby it was conceivable that the 
whole of the existing animal and vegetable worlds might have 
descended from at most avery few original forms. From a large 
array of facts drawn from observations made upon domestic plants 
and animals he inferred that from time to time slight peculiarities. 
due to differences of climate, &c., appear in the offspring which were 
not present in the parent, that these peculiarities may be transmitted 
to succeeding generations, especially where from their nature they 
are useful in enabling their possessors to maintain themselves in 
the general struggle for life. Hence varieties at first arising from 
accidental circumstances may become permanent, while the original 
form from which they sprang, being less well adapted to hold its 
own, perishes. Varieties become species and specific differences pass 
in the same way into generic. The most successful forms are by a 
process of “ natural selection’ made to overcome and survive those 
that are less fortunate. Hence the “survival of the fittest” is 
conceived to be the general law of nature. The present varied life 
of the globe may thus be explained by the continued accumulation, 
perpetuation, and increase of differences in the evolution, of plants _ 
and animals during the whole of geological time. Hence the 
geological record should contain a more or less full chronicle of the 
progress of this long history of development. 
It is now well known that in the embryonic development of 
animals there are traces of a progress from lower or more generalized — 
to higher or more specialized types. Since Mr. Darwin’s great work 
appeared, naturalists have devoted a vast amount of research to the 
subject, and have sought with persevering enthusiasm for any indica- 
tions of a relation between the order of appearance of organic forms 
in time and in embryonic development, and for evidence that species 
and genera of plants and animals have come into existence in the 
order which, according to the theory of evolution, might have been 
anticipated. 
It must be conceded that on the whole the testimony of the rocks is 
in favour of the doctrine of evolution. That there are difficulties still 
unexplained must be frankly granted. Mr. Darwin strongly insisted, 
and with obvious justice, on the imperfection of the geological reeord as 
one great source of these difficulties. Objections to the development 
theory may, as shown by Mr, Carruthers, be drawn from the observed 
