THISTLES. 103 



ready prickly fashion against the probahle assaults of their natural 

 enemies. They have forged darts, but have not learned to poison 

 them. Their prickly leaves and wings are amply sufficient for defense, 

 without the necessity for developing a virulent juice to be injected 

 into the very veins of their savage aggressors. Natural selection can 

 never push any special line of evolution further than is imperatively 

 called for by the wants and circumstances of the particular species. 

 It always necessarily leaves off just at the point where the protec- 

 tion afforded is fully sufficient to guard the kind from the possibility 

 of extinction. The thistles have found in actual practice that prickles 

 alone are quite enough to secure their boasted immunity from extra- 

 neous attacks : the nettles have practically discovered for themselves 

 that without stings they would soon be landed in the final limbo of 

 utter nonentity. 



Circumstances have still preserved for us a very tolerable series of 

 the successive stages whereby our existing thistles have gradually 

 acquired their present prickly and repellent characteristics. In the 

 good old days, while evolution was still fighting hard for public recog- 

 nition, it used to be urged by the uninstructed outsider that we never 

 found any " missing links." As a matter of fact, in ninety-nine cases 

 out of a hundred, the links are not and never were missing at all ; 

 and the practical difficulty is rather to establish any well-marked dis- 

 tinctions of kind than to discover long series of intermediate indi- 

 viduals. Just as the white man gradually merges into the negro by 

 slow steps, when we cross Europe, Asia, and Africa, through Italians, 

 Greeks, Levantines, Arabs, Egyptians, Nubians, Abyssinians, and 

 true Soudanese, so the various kinds of thistles merge imperceptibly 

 one into the other by innumerable varieties and natural hybrids. To 

 be sure, there are such things as well-marked species in nature ; but 

 there are also groups which it is impossible anywhere to split up into 

 good and distinctly different kinds. The brambles, the wild roses, the 

 St. John's-worts, and the epilobes absolutely defy regular classifica- 

 tion : the thistles, though perhaps a little more amenable to the subtile 

 arts of the artificial species-maker, still constantly glide one into the 

 other by strangely graduated intermediate forms. The great critx 

 really lies in the problem of the existence of such natural gradations ; 

 for, according to the strict Darwinian principles, the better adapted 

 and more specialized forms ought to crush out the intermediate types, 

 and leave the species well demarkated one from the other by broad in- 

 tervals. Probably the true explanation of the anomaly is to be found 

 in the wide distribution and high adaptability of these dominant forms ; 

 they can accommodate themselves exactly to such an extraordinary 

 variety and diversity of situations, that special intermediate types an- 

 swer best in every intermediate soil or climate. 



The most primitive and unarmed class of the thistle tribe is well 

 represented by the saw-wort of our copses, a true thistly plant in all its 



