388 THE EVOLUTION THEORY 



guard them against the heat of the deserts, the frost of the Arctic 

 resrions, or the malaria of the marshes into which their locomotive 

 blindly carries them. 



According to my view, the traveller — that is, tlie species — has 

 always a large choice of paths, and is able, even while he is on the 

 way, to discern whether he has chosen a right or a wrong one ; more- 

 over, in most cases, one or, it may Ije, a number of the paths lead to 

 the desired dwelling-place. But it also undoubtedl}' happens that, 

 after lono^ wanderino- and when nianv reo^ions have been traversed, 

 a company may finally arrive at a place Avhich is (|uite habital>le and 

 inviting at first sight, but which is surrounded on several sides by the 

 sea or by a rushing stream. As long as the soil remains fertile and 

 the climate healthy all goes well, but when matters change in this 

 respect, and perhaps the only way back lies through marshes and 

 desert land and is therefore impassable, then the colony will gradually' 

 die out — that is the death of the species. 



But let us now leave our parable and in([uire what paths the 

 organic world has actually taken in its transformations, in what 

 succession the individual forms of life have evolved from one anothei* ; 

 in short, how the actual genealogical tree of this earth's animate 

 population is really constructed in detail. To this I can only repl}^ 

 that we have many well-grounded suppositions, but only real certainty 

 in regard to isolated cases. Thus the genealogical tree of tlie horse 

 has been traced far back, and a great deal is known of the phylogen}' 

 of several Gastropods and Cephalopods, but in regard to the genea- 

 logical tree of organisms as a whole we can onl}' make guesses, many 

 of which are probable, but are never quite certain. The pala^onto- 

 logical records which the earth's crust has preserved for us for all the 

 ages are much too incomplete to admit of an}' certaint3\ Many 

 naturalists, notably Ernst Haeckel, have done good service in this 

 direction, for from what we know of palaeontology, embryology, and 

 morphology, they have constructed genealogical trees of the difterent 

 groups of organisms, which are intended to show us tlie actual suc- 

 cession of animal and plant forms. But, interesting as these attempts 

 are, they cannot for the most part be anything more than guess- 

 work, and I need not, therefore, state or discuss them here in any 

 detail, since they can afford us no aid in regard to the problem of the 

 origin of species with which these lectures are concerned. In regard 

 to the animal world at least — and the case of plants is probably very 

 similar — the record of fossil forms fails us at an early stage. Thus 

 the oldest and deepest strata in which fossils can be demonstrated, 

 the Cambrian formation, already contains Crustaceans, animals at 



