8 LOTSY, DE CHROMOSOMENGETALLEN DER ORGANISMEN. 
van belang dat de Mollusken en de Anneliden, die zoo nauw 
met elkaar verwant zijn, beide hetzelfde getal nl. 16 bezitten. 
Gedegenereerde of sterk gemodificeerde groepen nl. Trematodes 
en Acanthocephala hebben in den regel niet het type-getal van 
hun groep. 
De data voor de insecten zijn de volledigste en meest betrouw- 
bare en deze geven de beste gelegenheid om de veranderingen 
in het chromosomengetal na te gaan. 
Mrs. Harvey beproeft dit; waarbij zij van de volgende principes 
uitgaat, die ik woordelijk en onvertaald hier wedergeef: 
»A chromosome is really a compound structure, carrying many 
characters or genes which are themselves the elements of heredity. 
However genes may arise, it is conceivable that in some cases one 
Or more new genes may be placed in a chromosome without 
disturbing its integrity, the number of chromosomes in related 
species thus remaining the same. On the other hand such additional 
genes may disturb the existing complex and cause the whole mass 
of genes to be entirely redistributed, thus causing a change in 
chromosomenumber in nearly related species. Should a certain 
group of genes be placed in one chromosome in one species and 
in two in another, there would not be necessarily any difference 
in these two species. It would seem, however, that there might 
be a tendency in any large group of related animals for the genes 
to segregate out according to some definite pattern”. 
„If, therefore, we make a list of all the species of a certain 
class 1) of metazoa, leaving out of account results which are con- 
flicting or too old to be accurate, we find that a certain number 
of chromosomes is characteristic of that class; that is there are 
considerably more species having that number of chromosomes 
than any other number. This I will call the „type number”. The 
typenumber of a class is the most frequently occurring number 
and may be considered tentatively as the fundamental chromosome 
group”, 
„One or more chromosomes of this group or of a group derived 
from it may split into two (or more) parts, or they may fuse, thus 
% 
1) This term is used loosely to include related families, orders or classes 
of ordinary classification. 
