26 
well be imagined. Here, then, we must face this important fact, 
that until our knowledge is complete we cannot definitely fix our 
genera. Fixed genera, therefore, represent ideally a finality of know- 
ledge ; and as there is no such thing, it is logical to assume that a 
fixity of genera, and hence of generic terms, is utterly impossible. 
Such an apparent result would simply represent a hiatus in scientific 
progress, and hence something to be dreaded and deplored. 
In spite of this apparent instability, which indeed is only due to 
our ignorance, order and symmetry are the laws of nature. If it 
were possible to resurrect all extinct species, we should be able to 
construct a system perfect in its symmetry and in its detail, complete 
in its entirety, the position of every individual such that it would 
slide by imperceptible gradations into the parent from which it | 
originated. That harsh lines of demarcation are not always readily 
discernible between the several groups does not do away with the fact 
that the groups exist ; to assume that the allied divisions are perfectly 
separate and disconnected is to deny the first principles and axioms 
of evolution, and to break the chain on which the organic unity, as 
demonstrated by evolution, depends; whilst to assert that groups 
cease to be groups when they merge into each other would be to 
give away the scientific position altogether, and tend to suggest that 
the differences between two groups which merge on their outer 
limits are no longer worthy of investigation. ‘These close relation- 
ships, this merging of the most closely allied organic beings, one into 
another, must occur. It is the sole basis of evolution ; it exists not 
only between the larger and smaller groups of objects, but even 
between the individual objects themselves. It is the law that these 
changes should be slow and gradual, and the harsh lines of demarca- 
tion that exist are the exceptions, and it surely is not correct or 
logical to be guided by the exceptions instead of by the rule. Our 
groupings into families, sub-families, tribes, genera, and species are 
only an attempt to learn the arrangement which Nature has adopted, 
and to discover the lines along which her forces have moved. What 
we wish to find out are the natural genera or groups of organic 
beings, and in attempting to discover these we have to be careful to 
let all facts have their full value in determining what are really natural 
lines of relationship, and which only imaginary. 
From what has been said it will be gathered that genera are more 
or less natural groups of species collected about central nuclei, from 
which the various species have been evolved in the course of ages, 
the nucleus being as it were the central type. Since, however, any 
one of these species may, under favourable conditions, itself become 
a nucleus, and give rise to freshly divergent species, and form, as it 
were, a new genus, it is clear that the parent genus and offspring 
genus must merge into each other. It might be convenient occa- 
sionally to look upon such a secondary evolutionary group as 
contained in the first, and consider the whole a genus when the two 
parts merge insensibly into each other. 
