struggle between the parent species and more or less 
nascent varieties. 
When a whole species undergoes considerable variation, 
it becomes a new species. This seems to be assumed by- 
Mr. Wallace. He describes the process by which he 
conceives that the adaptation of a species to new con 
ditions may have been brought about. “ But,” he adds, 
“ it will now be a different creature.”* And it would 
seem to follow from this, that the ancestral form has 
become an extinct species ; so that, in this case, the 
extinction of species is the necessary prelude of the trans 
formation of species. A different creature would naturally 
be classified in a different species to that from which it 
had arisen, if we knew nothing of its origin. But will 
this knowledge make any difference ? It certainly does 
with some persons, who have provided us with very differ 
ent definitions of the word species. The old fashioned 
idea of species is that God made so many different kinds 
of animals to be fruitful inter se, to multiply and to repro 
duce offspring after their own typical likeness. The test 
of likeness is not, however, held very rigidly. It is 
assumed that a species may be variable but yet immut 
able, as indeed is the fact in some cases. But when, in 
spite of this latitude of interpretation, it is found that 
considerable changes take place in a given race, quite 
sufficient to constitute the offspring a new species—if we 
judge only by the comparative likeness or unlikeness—then 
the definition is modified altogether; the test of a true 
species is not to be found, we are told, in similarity or dis 
similarity, but in the continuous fertility of the offspring. 
Now, it is quite clear that these definitions exclude one 
another ; they are not capable of being harmonised. Exact 
U 
* Contributions, p. j/o.