55i 
in Times and Thuja. 
protoplasm derived from the parents tends to run through a specific cycle 
of changes, during which it transforms itself into an individual like the one 
of which it once formed a part/ but whether these changes are dominated 
by any one part of this protoplasm more than any other is still questioned 
by a few investigators. All writers recognize the fact that the transmission 
of hereditary characters is, from the physiological point of view, but the 
recurrence in successive generations of like forms of metabolism. It is 
not clear, however, to just what these like forms of metabolism are due. 
It has been suggested by Fick that every species is characterized by 
a protoplasm which differs from that of every other. There is, according 
to this view, a species or individual protoplasm which is handed on un- 
changed from generation to generation. The differentiation always takes 
place in such a way that the resulting individual resembles the original 
of which it was once a part. 
If it be true, as has generally been asserted by all students of biology, 
that the nucleus is the formative centre of the cell, that all processes of 
constructive metabolism are possible only through the nucleus, then it 
seems likewise evident that the same body may best be conceived of as 
responsible for the distinctive changes of metabolism which are manifested 
in the recurrence of definite characters, such as form, shape, and colour. 
So conceived, the idioplasmic theory is seen to rest on the assumption that 
the cytoplasm acting as the substratum of heredity is moulded and rendered 
specific by the idioplasmic characters of the nucleus. This relation which 
exists between cytoplasm and the nucleus must be regarded as taking 
place in accordance with definite chemical or molecular laws, always giving 
rise to the peculiar protoplasm characteristic of the species or of the 
individual. 
This theory does not deny the fact that the egg-cytoplasm fixes the 
type of development, or that the cytoplasm is a factor in the transmission 
of hereditary characters, but maintains that the action of the cytoplasm, 
whatever it may be, has been determined by the idioplasm. If such 
a view be accepted, the question arises as to whether the units of the 
idioplasm are characterized by different activities, or whether they merely 
act as part of the homogeneous substance. As already implied above, the 
weight of evidence seems to show that there are permanent elements of 
the idioplasm which are distinct in structure and function. Whether 
these material determinants differ in their chemical or molecular composi- 
tion is not known, but it seems that on this basis only can we conceive 
their true relation to be the transmission of hereditary characters. The 
students of cytology working hand in hand with the plant and animal 
breeders have established many of the fundamentals of heredity. It remains 
to be seen how much clearer the biological chemist will be able to render 
the conception. 
