CELLULAR STRUCTURE 31 



through which the ancestors of the higher 

 plants have almost certainly passed. They 

 may be regarded as morphological survivals 

 that have slipped out from the main stream 

 of evolution into the quiet backwaters of life, 

 preserving in themselves the types and forms 

 of a vegetation that otherwise might well 

 have passed into oblivion. Furthermore, 

 whilst they may be, and often are, admirably 

 fitted by their very simplicity and variety 

 to certain kinds of surroundings, they are 

 not suited for a life under other conditions 

 which demand a more highly specialised 

 body. 



As a matter of fact it is far from easy 

 to define very exactly what is meant by 

 44 higher " and " lower " types respectively. 

 We commonly associate the ideas of spe- 

 cialisation and differentiation with the higher 

 types. An obvious adaptation to a particular 

 environment is often taken as a sign of high 

 organisation, but in reality very many of the 

 extremely simple plants are admirably adapted 

 to their particular surroundings. Moreover, 

 we are acquainted with numerous species 

 which at the present time are simple because 

 they have lost the complexity of structure 

 formerly possessed by their ancestors. We 

 often speak of these as degraded forms ; but 

 parasites, which illustrate this point very 

 well, are frequently admirably adapted by 

 their very simplicity of structure to their 

 particular modes of life. In practice, 



