Phenomena appear, in a word, to be explicable on the ground of development. We have already seen that various leading animal forms represent stages in the embryotic progress of the highest—the human being. Our brain goes through the various stages of a fish's, a reptile's, and a mammifer's brain, and finally becomes human.

While the external forms of all these various animals are so different, it is very remarkable that the whole are, after all, variations of a fundamental plan, which can be traced as a basis throughout the whole, the variations being merely modifications of that plan to suit the particular conditions in which each particular animal has been designed to live. Starting from the primeval germ, which, as we have seen, is the representative of a particular order of full-grown animals, we find all others to be merely advances from that type, with the extension of endowments and modification of forms which are required in each particular case; each form, also, retaining a strong affinity to that which precedes it, and tending to impress its own features on that which succeeds.

The supporters of the Development Hypothesis... can show that any existing species—animal or vegetable—when placed under conditions different from its previous ones, immediately begins to undergo certain changes fitting it for the new conditions. They can show that in successive generations these changes continue; until, ultimately, the new conditions become the natural ones. They can show that in cultivated plants, in domesticated animals, and in the several races of men, such alterations have taken place. They can show that the degrees of difference so produced are often, as in dogs, greater than those on which distinctions of species are in other cases founded.

Should there not be manifest progress and development but in a higher sense than people have imagined it? ... No one is in his age alone, he builds on the preceding one, this becomes nothing but the foundation of the future, wants to be nothing but that — this is what we are told by the analogy in nature, God’s speaking exemplary model in all works! Manifestly so in the human species!

I suggest, then... that the first step was an advance under favor of peculiar conditions, from the simplest forms of being, to the next more complicated, and this through the medium of the ordinary process of generation.

Lamarck taught not only that species had been constantly undergoing changes from one geological period to another, but that there also had been a progressive advance of the organic world from the earliest to the latest times, from beings of the simplest to those of more and more complex structure and from the lowest instincts up to the highest, and, finally, from brute intelligence to the reasoning powers of Man. The improvement in the grade of being had been slow and continuous, and the human race itself was at length evolved out of the most highly organised and endowed of the inferior mammalia.

Another force driving progressive evolution is the so-called "arms-race." Prey animals evolve faster running speeds because predators do. Consequently predators have to evolve even faster running speeds, and so on, in an escalating spiral. Such arms races probably account for the spectacularly advanced engineering of eyes, ears, brains, bat "radar" and all the other high-tech weaponry that animals display.

Would it be too bold to imagine that, in the great length of time since the earth began to exist, perhaps millions of ages before the commencement of the history of mankind would it be too bold to imagine that all warm-blooded animals have arisen from one living filament, which the great First Cause endued with animality, with the power of acquiring new parts, attended with new propensities, directed by irritations, sensations, volitions and associations, and thus possessing the faculty of continuing to improve by its own inherent activity, and of delivering down these improvements by generation to its posterity, world without end!

Evolution proceeds on three general lines: the spiritual, the mental-emotional,and the astral-vital; and the physical body is the channel through which all these in wrapped capacities, tendencies, and powers, express themselves on the physical plane, if the environment at any particular moment or at any particular passage of time be appropriate and fit for the expression of this or that or of some other such attribute, power, or faculty. (Ch 10)

[W]e realise... the fact and... reason for the diversity of animal behaviour. From the moment we regard evolution as... psychical transformation, we see... a multitude of forms of instincts each corresponding to a... solution of the problem of life. The 'psychical' make-up of an insect is not and cannot be that of a vertebrate ; nor can... [that] of a squirrel be that of a cat or... elephant: this in virtue of the position[s]... on the tree of life. ...[W]e begin to see ...a gradation formed. If instinct is a variable dimension, the instincts will... create, beneath their complexity, a growing system. They will form as a whole a... fan-like structure in which the higher terms on each nervure are recognised... by a greater range of choice and depending on a better defined centre of coordination and consciousness. ...The 'psychical' make-up of a dog... is... superior to that of a mole or a fish. ...[T]he upholders of the spiritual explanation have no need to be disconcerted when they see... in the higher animals (particularly in the great apes) ways and reactions which strangely recall... 'a reasoning soul'. If the story of life is no more than a movement of consciousness veiled by morphology, it is inevitable that... in the proximity of man, the 'psychical' make-ups seem to reach the borders of intelligence.

To comprehend evolutionary Theory, one must first understand that it’s only ever a matter of changing proportions –altering or enhancing existing features to build on what is already there. Developmental biology, genetics, and comparative morphology combine to confirm many of these taxonomic stages such that organs do not seem to have appeared abruptly or fully-formed as if out of nowhere, because there is an implied evolutionary origin evident in every case. Even the transition of fish-to-tetrapods, dinosaurs-to-birds, or apes-to-men are each are just a matter of incremental, superficial changes being slowly compiled atop successive tiers of fundamental similarities. These represent monophyletic clades which will forever encompass all the descendants of that clade. This is why birds are still dinosaurs, and humans are still apes, and both are still stegocephalian chordates.

A history of animated nature must itself be animated.

It is interesting to contemplate an entangled bank, clothed with many plants of many kinds, with birds singing on the bushes, with various insects flitting about, and with worms crawling through the damp earth, and to reflect that these elaborately constructed forms, so different from each other, and dependent on each other in so complex a manner, have all been produced by laws acting around us. These laws, taken in the largest sense, being Growth with Reproduction; Inheritance which is almost implied by reproduction; Variability from the indirect and direct action of the external conditions of life, and from use and disuse; a Ratio of Increase so high as to lead to a Struggle for Life, and as a consequence to Natural Selection, entailing Divergence of Character and the Extinction of less-improved forms. Thus, from the war of nature, from famine and death, the most exalted object which we are capable of conceiving, namely, the production of the higher animals, directly follows. There is grandeur in this view of life, with its several powers, having been originally breathed into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved.

:(vi) … the "animal" level, characterized by increased mobility, teleological behavior and self-awareness...