HEREDITY, AND THE ORIGIN OF SPECIES. 27 
gained. It is suggested therefore that the conception of 
frequency of mutation is the primary idea, although the 
action might become intensified in certain periods, of more 
or less definite limits. 
Finally we may consider the causes inducing or aftect- 
ing mutations. Mutants are found to be the most numer- 
ous under conditions most favorable to the growth and re- 
production of the parental type, and this is also true of all 
anomalous structures. It is thus to be seen that mutations 
probably do not give rise to species most readily under the 
stress of unfavorable environment, or under any conditions 
which weaken the parental form, but when it is at a maxi- 
mum of activity. 
In the attempts to localize the changes in the cells, or 
in the chromosomes, which result in the formation of atypic 
individuals from seeds we are confident for theoretical rea- 
sons that these ensue previous to the reducing or qualita- 
tive divisions in the formation of the egg, or in the pollen 
mother-cells. Just what this change may be, we are un- 
able to say, and shall probably know no more about it until 
the cytologist shall have given us some clue to the manner 
in which the separate qualities are represented in the 
chromosomes, in which the mutative changes must ensue. 
It is self-evident that in a mutation, some characters or 
qualities being borne along steadily from cell to cell in the 
divisions must be thrown into a latent condition, or perhaps 
totally lost, while simultaneously or separately, other qual- 
ities may be acquired by what actual operation we do not 
know. It seems fairly obvious, however, that these salta- 
tions arising from the non-uniform action of the chromo- 
somes, must take place in response to some stimulus outside 
of the protoplast in which it actually occurs. This by no 
means supposes that the stimulation comes from climatic, 
or other environmental factors, but in all probability re- 
sults from enzymatic or other action from neighboring 
