"The birds which do not carry food to their young, and do not therefore marry, are armed with spurs for the purpose of fighting for the exclusive possession of the females, as cocks and quails. It is certain that these weapons are not provided for their defence against other adversaries, because the females of these species are without this armour. The final cause of this contest among the males seems to be _that the strongest and most active animal should propagate the species, which should thence become improved_."[180] Dr. Darwin would have been on stronger ground if he had said that the _effect_ of the contest among the males was that the fittest should survive, and hence transmit any fit modifications which had occurred to them as vitally true, rather than that the desire to attain this end had caused the contest; but either way the sentence just given is sufficient to show that he was not blind to the fact that the fittest commonly survive, and to the consequences of this fact. The use, however, of the word "thence," as well as of the expression "final cause," is loose, as Dr. Darwin would no doubt readily have admitted. Improvement in the species is due quite as much, by Dr. Darwin's own showing, to the causes which have led to such and such an animal's making itself the fittest, as to the fact that if fittest it will be more likely to survive and transmit its improvement. There have been two factors in modification; the one provides variations, the other accumulates them; neither can claim exclusive right to the word "thence," as though the modification was due to it and to it only. Dr. Darwin's use of the word "thence" here is clearly a slip, and nothing else; but it is one which brings him for the moment into the very error into which his grandson has fallen more disastrously.