294 MORPHOLOGY 
is the most difficult and perhaps the most important problem in biology. 
It means the transmission from parent to offspring of a similar structure, 
a transmission that involves fundamental resemblances with differences 
in detail. The possible machinery of heredity has been observed, but 
the factors controlling and determining the product are elusive as yet. 
Little more can be done than to state the problem. 
In the simplest plants and animals every cell has the power of repro- 
ducing the whole organism. In the more complex plants and animals 
this power is restricted, being retained only by the reproductive cells. 
It is evident, therefore, that the reproductive cells possess a very primitive 
power, a power that the other cells have lost. Reproductive cells are 
to be thought of not as cells that have acquired some special power, but 
as cells that have retained a primitive power that once belonged to all 
cells. All other kinds of cells may reproduce their own kind, but a 
reproductive cell can reproduce the whole organism. 
Reproduction is not simply cell multiplication, but also cell differentia- 
tion, and the organization of differentiated cells into organs, and of 
organs into the complete organism. Reproductive cells have been made 
to multiply cells under artificial stimulus; but it is this far-reaching direc- 
tive power that has baffled investigation. 
Any theory of heredity must explain not only likeness to the parent, 
but also variation from the parent and from every other individual, 
which is individuality. It must explain ancestral likeness, which is 
often called " atavism "; and also the sudden appearance of new char- 
acters, which after all may be very old ones. In short, the mass of obser- 
vations awaiting explanation by some law of heredity is enormous. 
The student of plant morphology is familiar with the general organiza- 
tion of a living cell. He has learned to recognize in the nucleus the prob- 
able machinery of heredity; and in the chromosomes the particular 
nuclear structure whose behavior suggests a definite relation to heredity. 
The mingling of paternal and maternal chromosomes in the fertilized 
egg is the beginning of a series of changes that must be followed even- 
tually; but as yet the particular chromosomes are lost to sight, and their 
fate is followed only by inference or imagination. Later, male and 
female chromosomes are recognized again, but only to function in re- 
production, and their varying influence in determining the structure of 
the individual remains unknown. 
