ii LIVING MATTEE 47 



of fossil plants or animals. Species, genera, families are seen to 

 disappear incontinently, and other species, other genera, other 

 families are substituted for them, with no evidence of that 

 continuity and regularity of development which is demanded by 

 the theory of evolution. Even when continuity of development 

 is observed for any given organ (e.g. the foot of Solidungula), it is 

 more apparent than real, since it has been arrived at by observing 

 a single organ apart from all the other organs which constitute 

 the species under consideration. 



Finally, it should not be forgotten that the fundamental 

 basis for a complete and satisfactory theory of the evolution of 

 the entire organic world, in virtue merely of the elements and 

 forces of the inorganic world (as the pure evolutionists maintain 

 with Spencer), is still wanting, viz. the demonstration of the 

 spontaneous generation of life from inorganic matter and force. 

 The greater the progress made by science, the more do the 

 organisms believed to be simple appear complex, and the more 

 improbable is spontaneous generation. 



Notwithstanding this and other serious difficulties, it must be 

 admitted that the hypothesis of evolution has proved in practice 

 to be a tool of remarkable utility. It has enabled us to gather 

 up under one concept an infinite variety of scattered facts which 

 would otherwise have escaped the researches and analysis of 

 modern science, and thanks to which our positive knowledge has 

 made extraordinary progress. 



Even if all biologists agree in admitting the theory of Evolu- 

 tion, this harmony ceases when we attempt to determine its 

 mechanism, i.e. its real causes and factors. 



The Darwinians and the so-called neo-Darwinians consider 

 natural selection to be the principal, if not the sole factor in 

 evolution, while the Lamarckians and the neo-Lamarckians almost 

 entirely deny the value of selection, and assert on the contrary 

 that transformation of species is the result of direct adaptation to 

 the variable conditions of environment. 



Darwin and all his modern followers, while they defend the 

 principle of selection to the hilt, are forced to admit an innate 

 tendency to variation within the species, without being able to 

 indicate its causes. If the said variation is slow, continuous, 

 gradual and indefinite, as supposed by Darwin, this does not 

 explain how the appearance of a variation can turn to the advan- 

 tage of the species, and give opportunity for selection, in such 

 a way as to favour the individual or individuals in which the new 

 variation originates, in the struggle for existence, to the prejudice 

 of the other individuals deprived of the same minimal variation. 



On the other hand, the concept of variability of species, both 

 in plants and animals, has made considerable progress. In the 

 time of Darwin a pure speculation, it is now a positive experi- 



