SPECIES AND GENEEA. 163 



species, which soon eliminates the cross, and leads to a complete 

 reversion to that species. 



The physiological test is consonant with morphological evidence. In- 

 dividuals of the same species are capable of indiscriminate fertilization 

 because they are exactly alike in all essentials of structure. In hybrids 

 produced between two species of a genus, the parents agree sufficiently in 

 structure to allow of their producing a few fertile seeds ; but the plants 

 raised from these seeds contain two contradictory impulses, which so far 

 prevent the perfection of their organization that they either remain barren 

 or a dissociation of the mixed characteristics occurs with, it may be, their 

 ultimate entire extinction. 



Origin of Species Selection. Supposing species to have originated 

 from a few primordial forms, from which all existing species have been 

 derived, just as individuals may be traced back to a common parent stock, 

 the question then arises as to what causes have produced the modifica- 

 tions. Where, on this hypothesis, there were originally a few, or perhaps 

 a single primordial form, to which all then existing individuals might 

 have been referred, there is now an infinite number of forms both in the 

 animal and vegetable kingdoms. How have these arisen ? To this 

 question the answer given by various naturalists has been different. 



By some the variations have been attributed to the influence of external 

 conditions; by Darwin to an innate tendency, producing variations of 

 structure, some of which, under given circumstances, would be favour- 

 able to the progress and development of the individual, and others not 

 so. In the battle of life, the struggle constantly going on in animated 

 nature, those variations most advantageous to the organism in its compe- 

 tition with others would be preserved by " natural selection," while 

 other variations of less advantageous character would be obliterated or 

 not perpetuated. Hence the victory would be to the strongest ; the 

 weakest would go to the wall, and the result would be, in Mr. Spencer's 

 language, " the survival of the fittest." It will thus be seen that on this 

 hypothesis species are not considered immutable, and variations, especially 

 such as are advantageous to the organisms, are regarded as the starting- 

 points of new species. With reference to these points the student will 

 do well to bear in mind that these and kindred speculations are not to be 

 treated as dogmas or creeds, but as means to an end, and that end the 

 more perfect knowledge of the origin and relation of existing forms. 

 Any hypothesis or theory which will serve to correlate and bind together 

 a number of otherwise isolated facts and explain their interdependence, 

 is valuable not only for what it effects at the time, but as a focus around 

 which other facts may in future be gathered. That hypothesis is best 

 which serves to give a rational explanation of the largest number of ob- 

 served phenomena of the greatest importance. Tried by this test, the 

 Darwinian hypothesis, or, rather, the theory of evolution, has great ad- 

 vantages, and presents on the whole fewer difficulties and less inconsis- 

 tencies than the older hypothesis of separate creation of each species. 

 Particularly does this seem true in the case of the subject now before us 

 the classification of plants. The admission of the principle of filiation 

 and genealogical descent gives the natural system of classification a clearer 

 claim to its title of " natural " than it had before, supplies the explanation 



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