At the dawn of every fresh manifestation, the Unknown Essence, which forms the One Reality, having completed its appointed period of rest, begins to activate itself once more, the One breaking up into myriads of Divine Sparks, each of which undertakes its long obligatory pilgrimage through the Cycle of Incarnation, garnering all possible experience in all the departments of Nature, ascending slowly and gradually "through all the degrees of intelligence, from the lowest to the highest Manas, from mineral and plant, up to the holiest archangel (Dhyani-Buddha)." These Eternal Pilgrims are called Monads. They are units of Atma-Buddhi which have acquired their own individuality by passing "through every elemental form of the phenomenal world of that Manvantara" until they came to the stage of self-consciousness and attained the stature and dignity of Manhood. Life in the lower Kingdoms of Nature is ever tending upwards towards the human stage, and therefore it is said that "every atom in the Universe has the potentiality of self-consciousness in it."
But let us consider at the outset some of the simplest definitions pertaining to this term MONAD, as that will perhaps help us in coming to a better understanding of this somewhat unfamiliar and intricate, though none the less important, metaphysical subject:
MONAD (Gr.). The Unity, the one; but in Occultism it often means the unified triad, Atma-Buddhi-Manas, or the duad, Atma-Buddhi, that immortal part of man which reincarnates in the lower kingdoms, and gradually progresses through them to Man and then to the final goal—Nirvana. (The Theosophical Glossary)
In our study of this subject we should first of all understand the distinction between the Monadic Essence which permeates all the lower kingdoms in Nature and which breaks up into individual Monads only in the human kingdom where self-consciousness first comes into play, endowing each individual human soul with its own free will and choice along with its accompanying moral responsibility. Therefore we say that animals have no individual souls, as we know human souls to be, and that their activity is the result of the degree of intelligence acquired by the Monadic Essence that is animating those particular forms of life. In other words:
The ocean (of matter) does not divide into its potential and constituent drops until the sweep of the life-impulse reaches the evolutionary stage of man-birth. The tendency towards segregation into individual Monads is gradual, and in the higher animals comes almost to the point....The "Monadic Essence" begins to imperceptibly differentiate towards individual consciousness in the Vegetable Kingdom. (I, 178-179)
So in the mineral, vegetable and animal kingdoms the progressive evolution of the Monadic Essence goes on from stage to stage until it comes to the human kingdom where the intelligence is unfolded sufficiently to enable it to be differentiated into individual self-conscious Monads with the help of the process of the "lighting up of Manas." So it is only when the human stage is reached that Atma-Buddhi, the Divine Spark, which has passed by means of natural impulse through all the lower kingdoms of Nature, becomes conjoined with Manas, appears on the scene as a Human Ego and undertakes its further "pilgrimage" as Atma-Buddhi-Manas, a self-conscious Individuality, through a long series of reincarnations in one human personality after another on this earth.
The Secret Doctrine explains to us that "there exists in Nature a triple evolutionary scheme...inextricably interwoven and interblended at every point...the Monadic (or spiritual), the intellectual, and the physical evolutions....Each is represented in the constitution of man, the Microcosm of the great Macrocosm; and it is the union of these three streams in him which makes him the complex being he now is." (I, 181)
On the other hand, in another place in the same book, H.P.B., in referring again to Monadic evolution and deploring the difficulty encountered in having to expound great metaphysical verities in any of the Western languages where there is such paucity of suitable philosophical terms, says:
The same difficulty of language is met with in describing the "stages" through which the Monad passes. Metaphysically speaking, it is of course an absurdity to talk of the "development" of a Monad, or to say that it becomes "Man."...It stands to reason that a Monad cannot either progress or develop, or even be affected by the changes of states it passes through. It is not of this world or plane, and may be compared only to an indestructible star of divine light and fire, thrown down on to our Earth as a plank of salvation for the personalities in which it indwells. It is for the latter to cling to it; and thus partaking of its divine nature, obtain immortality. (I, 174-75 fn.)
So this is indeed a highly metaphysical subject and to grasp and comprehend it a good deal of study and reflection is needed. But students of Theosophy cannot afford to neglect metaphysics, knowing what H.P.B. has said about it: "Outside of metaphysics no occult philosophy, no esotericism is possible." And we may just as well note here what she has said about occult philosophy and esotericism:
The exact extent, depth, breadth, and length of the mysteries of Nature are to be found only in Eastern esoteric sciences. So vast and so profound are these that hardly a few, a very few of the highest Initiates—those whose very existence is known but to a small number of Adepts—are capable of assimilating the knowledge. Yet it is all there, and one by one facts and processes in Nature's workshops are permitted to find their way into the exact Sciences, while mysterious help is given to rare individuals in unravelling its arcana....
But this is somewhat a disgression, and we shall now revert to the subject under consideration.
Monads may be roughly divided into three great classes: (1) the first class is made up of the most developed ones, the Lunar Gods or Pitris, who are able to pass in the very first Round through the mineral, vegetable and animal kingdoms and come up to the human form on Globe A in that first Round; (2) the second class consists of all those who are able to attain to the human stage by the middle of the fourth Round when "the 'Door' into the human kingdom closes"; (3) the third class is that of the laggards, so to speak, who, because of Karmic impediments, "find themselves so far behind that they will reach the human stage only at the close of the seventh and last Round. They will, therefore, not be men of this chain, but will form the humanity of a future Manvantara."
The above, however, is only a rough classification which refers only to three important stages or grades attained by the Monads at different periods in the progressive development of their intelligence. But if we go deeper into the subject, we shall find that
...the Monads cycling round any septenary chain are divided into seven classes or hierarchies according to their respective stages of evolution, consciousness, and merit....The time-spaces between the appearances of these hierarchies on any one Globe are so adjusted that when Class 7, the last, appears on Globe A, Class 1, the first, has just passed on to Globe B, and so on, step by step, all round the chain. (I, 171)
This leads us into thinking that the seven classes of Monads referred to may be those relating respectively to the seven great departments in Nature made up of the three classes of the invisible elemental lives and the Mineral, Vegetable, Animal and Human Kingdoms.
There is one remarkable characteristic about this Monadic Essence in that every monad acts as a faithful reflector of the impresses made upon it by every other. In the words of H.A. Bjerregaard, whom H.P.B. quotes in The Secret Doctrine (I, 630-31), "every monad is a living mirror of the Universe within its own sphere....In every monad, therefore, the adept may read everything, even the future. Every monad or Elemental is a looking-glass that can speak."
We may take note of one more point before we bring this rough sketch of a very vast and intricate subject to a close, and it is that when mankind first appeared on earth,
though all of one common origin, yet for reasons given their potentialities and mental capabilities, outward or physical forms, and future characteristics, were very different....Some superior, others inferior, to suit the Karma of the various reincarnating Monads which could not be all of de same degree of purity in their last births in other worlds. This accounts for the difference of races, the inferiority of the savage, and other human varieties. (II, 249)
We are made for co-operation, like feet, like hands, like eyelids, like the rows of the upper and lower teeth. To act against one another then is contrary to nature.