We must,in the first place,be prepared to prove that the supposed causes of the phenomena exist in nature;that they are what the logicians call 'vera causae'--true causes;--in the next place,we should be prepared to show that the assumed causes of the phenomena are competent to produce such phenomena as those which we wish to explain by them;and in the last place,we ought to be able to show that no other known causes are competent to produce those phenomena.If we can succeed in satisfying these three conditions we shall have demonstrated our hypothesis;or rather I ought to say we shall have proved it as far as certainty is possible for us;for,after all,there is no one of our surest convictions which may not be upset,or at any rate modified by a further accession of knowledge.It was because it satisfied these conditions that we accepted the hypothesis as to the disappearance of the tea-pot and spoons in the case I supposed in a previous lecture;we found that our hypothesis on that subject was tenable and valid,because the supposed cause existed in nature,because it was competent to account for the phenomena,and because no other known cause was competent to account for them;and it is upon similar grounds that any hypothesis you choose to name is accepted in science as tenable and valid.
What is Mr.Darwin's hypothesis?As I apprehend it--for I have put it into a shape more convenient for common purposes than I could find 'verbatim'in his book--as I apprehend it,I say,it is,that all the phenomena of organic nature,past and present,result from,or are caused by,the inter-action of those properties of organic matter,which we have called ATAVISM and VARIABILITY,with the CONDITIONS OFEXISTENCE;or,in other words,--given the existence of organic matter,its tendency to transmit its properties,and its tendency occasionally to vary;and,lastly,given the conditions of existence by which organic matter is surrounded--that these put together are the causes of the Present and of the Past conditions of ORGANIC NATURE.
Such is the hypothesis as I understand it.Now let us see how it will stand the various tests which I laid down just now.In the first place,do these supposed causes of the phenomena exist in nature?Is it the fact that in nature these properties of organic matter--atavism and variability--and those phenomena which we have called the conditions of existence,--is it true that they exist?Well,of course,if they do not exist,all that I have told you in the last three or four lectures must be incorrect,because I have been attempting to prove that they do exist,and I take it that there is abundant evidence that they do exist;so far,therefore,the hypothesis does not break down.
But in the next place comes a much more difficult inquiry:--Are the causes indicated competent to give rise to the phenomena of organic nature?I suspect that this is indubitable to a certain extent.It is demonstrable,I think,as I have endeavoured to show you,that they are perfectly competent to give rise to all the phenomena which are exhibited by RACES in nature.Furthermore,I believe that they are quite competent to account for all that we may call purely structural phenomena which are exhibited by SPECIES in nature.On that point also I have already enlarged somewhat.Again,I think that the causes assumed are competent to account for most of the physiological characteristics of species,and I not only think that they are competent to account for them,but I think that they account for many things which otherwise remain wholly unaccountable and inexplicable,and I may say incomprehensible.For a full exposition of the grounds on which this conviction is based,I must refer you to Mr.Darwin's work;all that I can do now is to illustrate what I have said by two or three cases taken almost at random.
I drew your attention,on a previous evening,to the facts which are embodied in our systems of Classification,which are the results of the examination and comparison of the different members of the animal kingdom one with another.I mentioned that the whole of the animal kingdom is divisible into five sub-kingdoms;that each of these sub-kingdoms is again divisible into provinces;that each province may be divided into classes,and the classes into the successively smaller groups,orders,families,genera,and species.
Now,in each of these groups,the resemblance in structure among the members of the group is closer in proportion as the group is smaller.
Thus,a man and a worm are members of the animal kingdom in virtue of certain apparently slight though really fundamental resemblances which they present.But a man and a fish are members of the same sub-kingdom 'Vertebrata',because they are much more like one another than either of them is to a worm,or a snail,or any member of the other sub-kingdoms.For similar reasons men and horses are arranged as members of the same Class,'Mammalia';men and apes as members of the same Order,'Primates';and if there were any animals more like men than they were like any of the apes,and yet different from men in important and constant particulars of their organization,we should rank them as members of the same Family,or of the same Genus,but as of distinct Species.
