In 1849 Professor Owen, in his treatise on "Parthenogenesis," put forward another conception.

According to this, the cells resulting from the subdivision of the germ-cell preserve their developmental force, unless employed in building up definite organic structures.

In certain creatures, and in certain parts of other creatures, germ-cells unused are stored up, and by their agency lost limbs and {218} other mutilations are repaired.

Such unused products of the germ-cell are also supposed to become located in the generative products.
According to Mr.Herbert Spencer, in his "Principles of Biology," each living organism consists of certain so-called "physiological units." Each of these units has an innate power and capacity, by which it tends to build up and reproduce the entire organism of which it forms a part, unless in the meantime its force is exhausted by its taking part in the production of some distinct and definite tissue--a condition somewhat similar to that conceived by Professor Owen.
Now, at first sight, Mr.Darwin's atomic theory appears to be more simple than any of the others.

It has been objected that while Mr.Spencer's theory requires the assumption of an innate power and tendency in each physiological unit, Mr.Darwin's, on the other hand, requires nothing of the kind, but explains the evolution of each individual by purely mechanical conceptions.