With this bird’s-eye view of the course of Development, you will be able to appreciate the grand law first clearly enunciated by Goethe and Von Baer, as the law of animal life, namely, that Development is always from the general to the special, from the simple to the complex, from the homogeneous to the heterogeneous; and this by a gradual series of _differentiations_.[26] Or to put it into the music of our deeply meditative Tennyson:-- “All nature widens upward. Evermore The simpler essence lower lies: More complex is more perfect--owning more Discourse, more widely wise.” You are now familiarized with the words “differentiation” and “development,” so often met with in modern writers; and have gained a distinct idea of what an “organ” is; so that on hearing of an animal without organs, you will at once conclude that in such an animal there has been no setting apart of any portion of the body for special purposes, but that all parts serve all purposes indiscriminately. Here is our Opalina, for example, without mouth, or stomach, or any other organ. It is an assimilating surface in every part; in every part a breathing, sensitive surface. Living on liquid food, it does not need a mouth to seize, or a stomach to digest, such food. The liquid, or gas, passes through the Opalina’s delicate skin, by a process which is called _endosmosis_; it there serves as food; and the refuse passes out again by a similar process, called _exosmosis_. This is the way in which many animals and all plants are nourished. The cell at the end of a rootlet, which the plant sends burrowing through the earth, has no mouth to seize, no open pores to admit the liquid that it needs; nevertheless the liquid passes into the cell, through its delicate cell-wall, and passes from this cell to _other_ cells, upwards from the rootlet to the bud. It is in this way, also, that the Opalina feeds: it is all-mouth, no-mouth; all-stomach, no-stomach. Every part of its body performs the functions which in more complex animals are performed by organs specially set apart. It feeds without mouth, breathes without lungs, and moves without muscles.