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 
“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, 
Wwe 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, 
