458 Ever-sporting Varieties 



ternal conditions I might point to the genus 

 Acacia. As we have seen in a previous 

 lecture some of the numerous species of this 

 genus bear bi-pinnate leaves, while others 

 have only flattened leaf-stalks. According to 

 the prevailing systematic conceptions, the 

 last must have been derived from the first by 

 the loss of the blades and the corresponding 

 increase of size and superficial extension of the 

 stalk. In proof of this view they exhibit, as we 

 have described, the ancestral characters in the 

 young plantlets, and this production of bi-pin- 

 nate leaves has probably been retained at the pe- 

 riod of the corresponding negative mutations, 

 because of some distinct, though still unknown 

 use. 



Summarizing the results of this discussion, 

 we may state that useful dimorphism, or dou- 

 ble adaptation, is a substitution of characters 

 quite analogous to the useless dimorphism of 

 cultivated ever-sporting varieties and the stray 

 occurrence of hereditary monstrosities. The 

 same laws and conditions prevail in both cases. 



