LECTURE XXXVI 



SPONTANEOUS GENERATION AND EVOLUTION: 



CONCLUSION 



Sjjontaneous generation — Experimental tests impossible — Only the lowest and 

 smallest forms of life can be referred to spontaneous generatioxi— Chemical postulates 

 for spontaneous generation — Empedocles modernized — The locality of spontaneous 

 generation — Progress of organization — Direct and indirect influences caiising variation 

 — The various modes of selection — Everything depends upon selection — Sinking from 

 heights of organization already attained — Paths of evolution — Tlie forces effecting it — 

 Plasticity of living matter— Predetermination of the animate world— Many-sided 

 adaptation of eacli group — Aquatic mammals and insects, parasites — Niigeli's variation 

 in a definite direction — Analogy of the traveller — Genealogical trees — The diversity of 

 forms of life is unlimited — Tlie origin of tlie purposeful apart from purposive forces 

 woi'king towards an end — The limits of knowledge^Limitation of the human intelli- 

 gence by selection — Human genius— Conclusion. 



We have now reached the end of our studies, and they have 

 given us satisfaction, at least in so far that they have brought us 

 certainty in regard to the chief and fundamental question which can 

 be asked in reference to the origin of the modei-n animate world 

 of organisms. There remains no doubt in our minds that the theory 

 of descent is justified ; we know, just as surely as that the earth goes 

 round the sun, that the living world upon our earth was not created 

 . all at once and in the state in which we know it, but that it has 

 gradually evolved through what, to our human estimate, seem enor- 

 mously long periods of time. This conclusion is now firmly estab- 

 lished and will never again become doubtful. The assumption, too, 

 that the more lowly organisms formed the beginnings of life, and 

 that an ascent has taken place from the lowest to the higher and 

 highest, has become to our minds a probability verging upon certainty. 

 But there remains one point which we have not yet touched upon — 

 the problem of the origin of these first organisms. 



There are only two possibilities : either that they have been borne 

 to our earth from outside, from somewhere else in the universe, or that 

 1 they have originated upon our earth itself through what is called 

 ' spontaneous generation ' — generatio spontanea. 



The idea that very lowly living organisms might have been 

 concealed within the clefts and crevices of meteorites, and might thus 



