144 



THE FACTORS 



[Part i 



At the phylogenetic commencement of symbiosis, the ants bored an 

 entrance through the groove, evidently because the wall was somewhat 

 thinner there, and in particular, in accordance with a custom that is 

 almost always followed and is connected with the domestic arrangements, 

 they bored as much as possible in the upper part of their dwelling. All 

 features that facilitate boring through this place must have been retained 

 in the struggle for existence, and been further added to through selection '. 

 They finally led to the differentiation of the thin weak diaphragm that 

 has been described. 



Fig. 79. Cecropia adenopus. Part of a transverse section of the wall of an internode, showing 

 the diaphragm. Slightly enlarged. 



The ants of the Cecropia devote themselves in their dwellings to thij 

 tending of Aphidae ; they would seldom leave this work, and would rarely 0. 

 never visit the foliage, were it not that the latter merited continued attentior 

 The base of the petiole is covered on its dorsal surface with a brow 

 velvety coating upon which, in uninhabited trees, ovoid whitish bodie: 

 about 2 mm. long, lie quite loose (Fig. 80). The presence of such bodie; 

 which are named Midler's corpuscles after Fritz Miiller their discovere 

 is a sure sign that the tree is uninhabited, and they are always visibl 

 in our conservatories. They are absolutely wanting on the surface c 

 the pulvinus of inhabited trees, because they are continually carried awa 

 and eaten by the ants that are always looking for them, just as in Acaci 

 cornigera. Muller's corpuscles, like those of Belt, consist of delicat 

 parenchyma rich in proteids and oil. 



' I have observed on the mountains between the provinces Rio and JMinas a species ' 

 Cecropia which appears to have attained a lower stage of adaptation, since, among othi 

 points, the entrance is absent in younger plants. The observations, however, were bi 

 fragmentary. 



