545 
in Pinus and Thuja . 
to the fact that the chromosomes have retained throughout the life of the 
organism down to spore-formation their individuality, and have simply 
become associated in pairs during meiosis. 
Less certain evidence is that derived by Farmer (’07) for hybrid ferns. 
In Polypodium Schneideri , the result of a cross between P. aureus and 
P. elegantissumtis , the chromosome number at meiosis is not a mean between 
that of the two species from which it was derived, but is considerably larger. 
There are in this case also several unpaired members. The large number 
of chromosomes of this hybrid, as well as the original species from which it 
was derived, renders the results less convincing than that of the Drosera 
hybrid. 
Another piece of evidence which has had great influence on this 
favourite hypothesis is the presence in certain insects of differential chromo- 
somes which are so different from all others that they may be followed 
readily from one division to another. It is, in fact, to this phenomenon 
more than to any other that we are largely indebted for our present view 
of synapsis and reduction, first suggested by Montgomery (’01) and later 
developed by Sutton and Boveri. 
Wilson (’05) concludes from a study of these differentials that the 
chromosomes are ‘ definite, well characterized entities which show the most 
marked individual characteristics of behaviour, which in some manner persist 
from one cell generation to another without loss of their specific character, 
and which unite in synapsis and are distributed in the next ensuing 
maturation division in a perfectly definite manner’. This writer regards, 
therefore, the question of the individuality of the bodies in question as 
definitely settled. He considers the problems of cytology to be ‘ problems 
of the comparative morphology and physiology of the chromosomes with 
the ultimate aim of attempting their specific correlation with the phenomena 
of heredity and development’. Wilson himself, working on this hypothesis, 
has developed his well-known theory of sex determination. But in the 
plant world, although differences have often been noted in respect to both 
size and shape of chromosomes, no such differentials occur as are found 
in the cases cited above. Cardiff (’06) has suggested that it is highly 
probable that one chromosome in Acer is structurally different from the 
others, but he does not offer any speculation as to its significance. Like- 
wise Gates (’07) ventures the assertion that a differential occurs in Oenothera. 
Full confidence cannot be placed in this work, however, until it is more 
certainly reported and definitely confirmed for many genera of plants, 
a condition which it seems safe to predict will never be realized. In 
almost any spore-mother-cell the chromosomes differ in form and size, but 
that this difference is constant enough to warrant any definite conclusions 
concerning their correlation with heredity and development does not seem 
probable. 
pp 
