There are many substances in nature of the same size that would support as large a head as a cabbage.
The reasons of the mortality of plants are at present perfectly unknown to us.

No man can say why such a plant is annual, another biennial, and another endures for ages.

The whole affair in all these cases, in plants, animals, and in the human race, is an affair of experience, and I only conclude that man is mortal because the invariable experience of all ages has proved the mortality of those materials of which his visible body is made: What can we reason, but from what we know?
Sound philosophy will not authorize me to alter this opinion of the mortality of man on earth, till it can be clearly proved that the human race has made, and is making, a decided progress towards an illimitable extent of life.

And the chief reason why I adduced the two particular instances from animals and plants was to expose and illustrate, if I could, the fallacy of that argument which infers an unlimited progress, merely because some partial improvement has taken place, and that the limit of this improvement cannot be precisely ascertained.
The capacity of improvement in plants and animals, to a certain degree, no person can possibly doubt.

A clear and decided progress has already been made, and yet, I think, it appears that it would be highly absurd to say that this progress has no limits.