64 
R. Ruggles Gates. 
MUTATIONS AND EVOLUTION. 
By R. Ruggles Gates. 
CHAPTER II.— Continued. 
Foundations of the Mutation Concept. 
ROM the standpoint of the mutation concept, CEnothera lata, 
while a critical case in demonstrating the occurrence of 
germinal changes, differs in several respects from the most frequent 
and widespread type of mutation, i.e ., the type which results in a new 
Mendelian character. The latter type of change will be considered 
in greater detail in connection with the origin of dominant and 
recessive Mendelian characters, mass mutations and parallel 
mutations. The type of mutation involving an extra chromosome 
differs from this in the following points. (1) There are present 
three representatives of the extra chromosomes instead of a pair. 
A Mendelian mutation, on the other hand, will first appear usually 
as a heterozygous individual if the new character is dominant, or as 
an individual homozygous for the new character if it is recessive: 
that is, a pair of chromosomes only will be involved, one or both of 
which carry the new factor. (2) As we shall see later, the presence 
of the extra chromosome determines, at least in large measure, the 
hereditary behaviour of the form possessing it. (3) Chromosome 
duplication, viewed as a germinal change, arises through, or at 
least in connection with, a rearrangement of the chromatic material, 
resulting from an irregular distribution or non-disjunction of a 
pair of chromosomes in the heterotype mitosis, while when a 
Mendelian character arises one must assume it is by an alteration 
in one element of a chromosome. 
Tims while the bearing of the extra chromosome in (Enothera 
lata upon the mutation concept is clear in that, as we have already 
pointed out, there is a constant relation between the lata characters 
and the additional chromosome, yet the relation of this type to the 
numerous other forms now known to have 15 chromosomes is by 
no means so clear, and raises a number of intricate problems. 
These forms and the problems connected with them will be 
discussed later. For the moment we are concerned in pointing out 
some of the other bases of present concepts regarding mutations. 
The gigas and semigigas series of forms, having respectively 
28 and 21 chromosomes, again clearly justify the concept of a 
mutation as a cell change involving a new nuclear structure 
perpetuated by mitosis. This type of change is now known, 
