A more detailed account of the functions and structure of the cell must be reserved for the next chapter. In considering these and all other phenomena of vitality let me again recall the warning expressed in the taunt of Mephistopheles to the young student: the lines are as true to-day as they were when Goethe wrote them over a hundred years ago:—
“If some living thing you would learn about, You begin by driving its Spirit out; There lie the parts of it, one by one, But the binding Spirit, alas, is gone!”
Immense have been the preparations for me, Faithful and friendly the arms that have help’d me. * * * * * “Before I was born out of my mother generations guided me, My embryo has never been torpid, nothing could overlay it.”
There are two functions of organic life which are often confused together, but which it is well to keep distinct in our thought. These are Growth and Development. The mark of growth is that an organism, by assimilation from the outside world, becomes _larger than it was_. But in development it becomes _different from what it was_. The history of an embryo in the womb presents a succession of phenomena which, when one comes to realize them, almost stagger thought; for, while remaining the _same thing_ all through, it is continually becoming a _different class of thing_—first two cells, then one cell, then a fish, a quadruped, ultimately a human being. This is Development. Once born, it is laid hold of by the principle of Growth which lasts until maturity. Now in the groups called Species, as well as in individuals, we observe exactly the same distinction. The members of a species multiply and increase their numbers. This is Growth. But under certain conditions, which we have now to investigate, they vary in type and ultimately give rise to new species differing widely from that from which they sprang. This we call Development or, in the more popular term for the process when applied to species, Evolution.
The investigation of this process in all its details has been the master-impulse of biology ever since the fact of the process was established by the researches of Darwin.
In Darwin’s time the study of evolution was mainly an affair of what is called Natural History But it has now been realized that fully to comprehend the processes involved—so far as they can ever be comprehended—it is necessary to find out of what kind of material living beings are composed, and how their fundamental processes take place. “The ultimate problems of sex, fertilization, inheritance, and development,” says Wilson, have been now “shown to be _cell-problems_.”[26] Before going further, therefore, we must give some account of the leading facts connected with the structure and vital action of the cell.