4 8 
ture from parent to offspring, and therefore incapable of producing 
new characters ; so that no matter how highly any particular 
characteristic may be developed during the lifetime of an individual 
organism, that characteristic cannot possibly be inherited by the 
offspring, at birth that is to say. Hence they deny the correctness 
of part of the theory of development propounded by Darwin, 
who believed that the inheritance of acquired characters was one 
of the means through which new species arose, besides the crossing 
of one species or variety with another. All depends, however, on 
the sense in which the characters called “ acquired ” have arisen. 
If acquired characters were heritable in the sense that the progeny 
at birth would be provided with them, then an animal or a plant 
at various stages of its course through life ought to produce des- 
cendants showing characteristics corresponding to those acquired 
by the parents at particular stages. Although the traits evolved 
in an organ or tissue by the action of particular stimuli brought 
to bear on them in their environment may not appear in the 
progeny at their birth, there is at least no reason to suppose that 
the descendants should be less capable of developing the same 
characteristics in a similar way. On the contrary, each successive 
generation subjected to the same conditions — to the same stimuli 
or the same absence of stimuli — may carry the development of a 
characteristic further and further to an indefinite degree, and the 
capacity for such specialisation may become greater and greater 
the longer the favouring conditions keep the same. We can 
scarcely impose a limit to the degree to which an organ may 
develop under special stimuli, or to the successive forms an organ 
may pass through in a long series of generations, all under the 
action of the vital forces inherent in the germ cells. If an entirely 
new organ or a structure of a different type were supposed to be 
included in the expression “ acquired character,” it would be quite 
reasonable to doubt the possibility of such being produced in the 
individual or inherited by the descendants. But if the specialisa- 
tion has taken place simply from the action of the stimuli encoun- 
tered in the environment on the qualities inherent in the germ 
cells of the organism, it seems quite reasonable to expect that 
not only the capacity but the structure evolved on typical lines 
may in time become fixed and heritable ; and when by the action 
of the external forces (such as drought) educing these new traits 
those organisms lacking them are cut off on account of unfitness, 
the characters of those surviving may be very unlike the originals. 
Apart from the direct inheritance of acquired characteristics, 
however, a highly developed trait may have an important influence 
on the capacity or chances of an individual plant or animal so 
provided for effective crossing with another variety. 
The structural modifications by means of which plants are 
enabled to live and flourish in dry climates are very varied and 
interesting. In some cases the plant as a whole, or particular 
