1917.] Ecology of the Nilgiri and Pulney Highland Plateaus. 259 



vegetative parts are almost exactly the similar. Between the 

 two species of Aerides, in fact, the only real difference lies in the 

 shape of the flowers. Taken in conjunction with what has been 

 pointed out in regard to the two species of Pimpinella and of 

 Osbeekia, of which one in each case grows on the drv hill sides, 

 the other only in damp and sheltered spots, and with what is well 

 known of the many other species which occur only in special situa- 

 tions, it would seem that in the diagnosis of species greater stress 

 might well be laid on physiological characteristics of this kind, 

 not indeed to the exclusion of morphological characters but in 

 addition to them. In this connection it may be pointed out that 

 if there is considerable latitude in the conditions under which 

 many plants will thrive in nature, so also is there in the habit 

 of the plant the shape and hairiness of the leaves and in the 

 other external features on which species are founded. Much of 

 the work of Herbaria is in the critical re -examination of species 

 which have been lumped together or separated by previous 

 workers. And it must be conceded that the differences be- 

 tween species is ultimately a physiological one, and that the 

 morphological characters are but expressions of the living sub- 

 stance within. Physiological differences cannot, of course, be 

 determined in the herbarium where ultimately the critical deter- 

 mination of species must be made, though they could be made 

 use of there if known ; but for the identification of species in 

 the field and their recognition from printed descriptions, the in- 

 clusion, as part of the diagnosis of such characters, would often 

 be of great assistance to the collector. And reciprocally were 

 more stress laid on these, more notice would be taken of them 

 by collectors and advance on Ecological lines would be rendered 

 more rapid and secure. 







