540 Engler . — Plants of the Northern Temperate Zone. 
when the seeds find place and conditions suited for germination and 
climatic conditions for their development, which are not too different from 
those of their former native country. 
I am convinced that in such cases the somewhat different climate is 
the cause of all or at least of a part of the modifications. Sometimes in 
connexion with these new variations are also to be observed (cf. Cera- 
stium caespitosum ), which may become the beginning of other new forms. 
The constancy of such climatical adaptations may be a different one 
and often become fixed through a geological period. I may add that 
systematic studies have also convinced me that many of the xerophytes 
must have originated from mesophytes, and that a good deal (I do not say 
all) of the qualities of xerophytes, which are usually called adaptations for 
protection against a dry climate, are caused by this climate itself (cf. 
A. Engler, Pflanzenwelt von Deutsch-Ostafrika, A, p. 25)- 
