These are Naegeli's views, and they have been elaborated by Pflüger in his important treatise on The Teleological Mechanism of Living Nature (1877).
Vöchting writes in similar fashion:
'In a tree that is growing under normal conditions, without being subjected to injury, all the organs appear in definite relation to each other: so many leaves correspond to a definite number of twigs and branches. These spring from a stem of proportionate thickness, and the stem passes into a definitely proportioned tap-root, from which arise a due array of lateral roots. In normal conditions all these organs are in equilibrium. An apple-tree, growing on the line where tilled garden ground meets a lawn, grows more vigorously on the side towards the garden. If one of the roots of an apple-tree with three main roots and three branches be amputated, then the corresponding branch will lag behind in growth, although it may not absolutely perish.' 'The equilibrium varies according to the specific nature of the tree. It is shown in one way in the oak, in another in the beech, and is different in the varieties of a species.'
Finally, consider this statement from Goebel's Treatise on the Morphology and Physiology of the Leaf: 'The fact that lateral buds do not develop while the axial bud is still growing vigorously depends upon the relation between the two. That I denote as correlation of growth.'
The dependence of parts upon each other, and upon the whole, is specially clear and instructive in cases where different plant individuals are united by budding or grafting. To limit the growth of a tree, and to induce it to become dwarfed, it is necessary only to graft it upon a nearly allied but dwarf variety. When a pear-tree is grafted upon the quince, which is characterized by its dwarf-like growth, the vegetative growth of the pear is reduced exceedingly. It produces shorter and weaker shoots; all the dwarf varieties of the pear employed as wall fruits, or growing into the little pyramids spoken of in the trade as 'cordon'-trees and potting-trees, could not have been produced unless the gardener had had the quince as a natural dwarf stock (Vöchting). With the dwarfing is associated a freer and earlier production of fruit. Other kinds of fruit-trees, apples, apricots, and so forth, show the same course.
'The capacity to withstand external influences and the duration of life may be altered in the same way. The pistachio (Pistazia vera), cultivated in Frankfort, which is destroyed by a temperature lower than 7.5 degrees of frost, will survive 12.5 degrees if it has been grafted upon P. terebinthus. Moreover, when it is grown from a seedling, it may reach the age of 150 years; but when it has been grafted upon P. terebinthus its length of life is increased to 200 years; while, grafted on P. lentiscus, it reaches only about 40 years' (Vöchting).
Vöchting's experiments upon beetroot are still more characteristic. 'The stem of a beet plant that bore young buds gave rise to vegetative shoots when it was united with a young, still growing root, but to a blossoming stem when it had been grafted, in spring, upon an old root.'
Similarly, animal growth is correlative in all its stages. When a muscle becomes unusually large it sets up corresponding correlations of growth in many other parts of the body. The bloodvessels and nerves supplying it become larger, and the increase in the nerves leads to corresponding increase in the nerve centres. The tendons of origin and of insertion, and the parts of the skeleton to which these are attached, must react to the increased size of the muscle by growing larger; in fact for all the parts of the animal body the conclusions which Naegeli and other physiologists drew from plants are applicable. All the different elements of the body are in definite and intimate touch with each other.
This is shown most beautifully and clearly in the extraordinarily interesting phenomena called dimorphism and polymorphism. These seem to me to show how very different final results may grow from identical rudiments, if these, in early stages of development, be subjected to different external influences.
Finally, I have a little to say about the sexual dimorphism that occurs so generally in the animal kingdom.