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-probandi of myrmecophily on the shoulders of him who affirms 

 its existence ? 



But when answered in this manner, the advocate of myrme- 

 cophily shifts his ground and says that the acacias have obvi- 

 ously survived a multimillennial struggle with enemies which are 

 now either much reduced in numbers and hostility, or have alto- 

 gether perished, and that it was during this struggle that the , 

 plants were compelled to hold out inducements to an emmet 

 body-guard, which is perhaps at the present time nothing more 

 than a useless survival. Such arguments, in the face of our com- 

 plete ignorance of the past history of the ant acacias, may be 

 passed over without comment, or by mereh' pointing to the fact 

 that there are in Central America and Mexico many other species 

 of Acacia and acacia-like plants, with equall3' delicate foHage, 

 and growing in the same localities, but without enlarged thorns, 

 extrafloral nectaries, food-bodies, and attendant ants. Why have 

 these more numerous and more defenceless species survived 

 without the assistance of the Pseudomyrmas ? 



I admit that the thorns, extrafloral nectaries, and food-bodies 

 are pecuHar structures requiring an explanation. But such an 

 explanation should first be sought along physiological lines. 

 Madame von Üxküll-Güldenstern (1907) has recently shown 

 that botanists are still as ignorant as they were in the days of 

 Linné in regard to the function of the extrafloral nectaries of 

 plants in general. And the significance of such structures as the 

 Beltian bodies of the acacias, the Miillerian bodies of the Cecropias, 

 and the " bead-glands " of these and many other tropical plants 

 rarely or never visited by ants, is even more obscure. The 

 extraordinary enlargement of the thorns of the ant acacias, 

 especially of the African species, in which they are sometimes 

 so voluminous that the Crematogasters have to construct carton 

 partitions across their cavities, in order to convert them into 

 suitable nests, also suggests that these organs have some unknown 

 significance in the life of the plant, quite apart from their suit- 

 ability as dwe lungs or as supplies of food for the ants. 



The whole matter becomes clearer, I believe, when we turn 

 to the acacia ants themselves, for there can be no doubt that these 

 are exquisitely adapted to the plants. We have seen that four of 

 the Pseudomyrmas occur in no other situations, and that they 



