But if,on the other hand,you look on all forms of organized beings as the results of the gradual modification of a primitive type,the facts receive a meaning,and you see that these older conditions are the necessary predecessors of the present.Viewed in this light the facts of palaeontology receive a meaning--upon any other hypothesis,I am unable to see,in the slightest degree,what knowledge or signification we are to draw out of them.Again,note as bearing upon the same point,the singular likeness which obtains between the successive Faunae and Florae,whose remains are preserved on the rocks:you never find any great and enormous difference between the immediately successive Faunae and Florae,unless you have reason to believe there has also been a great lapse of time or a great change of conditions.
The animals,for instance,of the newest tertiary rocks,in any part of the world,are always,and without exception,found to be closely allied with those which now live in that part of the world.For example,in Europe,Asia,and Africa,the large mammals are at present rhinoceroses,hippopotamuses,elephants,lions,tigers,oxen,horses,etc.;and if you examine the newest tertiary deposits,which contain the animals and plants which immediately preceded those which now exist in the same country,you do not find gigantic specimens of ant-eaters and kangaroos,but you find rhinoceroses,elephants,lions,tigers,etc.,--of different species to those now living,--but still their close allies.If you turn to South America,where,at the present day,we have great sloths and armadilloes and creatures of that kind,what do you find in the newest tertiaries?You find the great sloth-like creature,the 'Megatherium',and the great armadillo,the 'Glyptodon',and so on.And if you go to Australia you find the same law holds good,namely,that that condition of organic nature which has preceded the one which now exists,presents differences perhaps of species,and of genera,but that the great types of organic structure are the same as those which now flourish.
What meaning has this fact upon any other hypothesis or supposition than one of successive modification?But if the population of the world,in any age,is the result of the gradual modification of the forms which peopled it in the preceding age,--if that has been the case,it is intelligible enough;because we may expect that the creature that results from the modification of an elephantine mammal shall be something like an elephant,and the creature which is produced by the modification of an armadillo-like mammal shall be like an armadillo.
Upon that supposition,I say,the facts are intelligible;upon any other,that I am aware of,they are not.
So far,the facts of palaeontology are consistent with almost any form of the doctrine of progressive modification;they would not be absolutely inconsistent with the wild speculations of De Maillet,or with the less objectionable hypothesis of Lamarck.But Mr.Darwin's views have one peculiar merit;and that is,that they are perfectly consistent with an array of facts which are utterly inconsistent with and fatal to,any other hypothesis of progressive modification which has yet been advanced.It is one remarkable peculiarity of Mr.
Darwin's hypothesis that it involves no necessary progression or incessant modification,and that it is perfectly consistent with the persistence for any length of time of a given primitive stock,contemporaneously with its modifications.To return to the case of the domestic breeds of pigeons,for example;you have the Dove-cot pigeon,which closely resembles the Rock pigeon,from which they all started,existing at the same time with the others.And if species are developed in the same way in nature,a primitive stock and its modifications may,occasionally,all find the conditions fitted for their existence;and though they come into competition,to a certain extent,with one another,the derivative species may not necessarily extirpate the primitive one,or 'vice versa'.
Now palaeontology shows us many facts which are perfectly harmonious with these observed effects of the process by which Mr.Darwin supposes species to have originated,but which appear to me to be totally inconsistent with any other hypothesis which has been proposed.There are some groups of animals and plants,in the fossil world,which have been said to belong to "persistent types,"because they have persisted,with very little change indeed,through a very great range of time,while everything about them has changed largely.There are families of fishes whose type of construction has persisted all the way from the carboniferous rock right up to the cretaceous;and others which have lasted through almost the whole range of the secondary rocks,and from the lias to the older tertiaries.It is something stupendous this--to consider a genus lasting without essential modifications through all this enormous lapse of time while almost everything else was changed and modified.
Thus I have no doubt that Mr.Darwin's hypothesis will be found competent to explain the majority of the phenomena exhibited by species in nature;but in an earlier lecture I spoke cautiously with respect to its power of explaining all the physiological peculiarities of species.
There is,in fact,one set of these peculiarities which the theory of selective modification,as it stands at present,is not wholly competent to explain,and that is the group of phenomena which Imentioned to you under the name of Hybridism,and which I explained to consist in the sterility of the offspring of certain species when crossed one with another.It matters not one whit whether this sterility is universal,or whether it exists only in a single case.
