I don’t know if by the time of writing the Fourfold root of the Principle of Sufficient Reason, Darwin had published his Origin*. In this book he criticizes De Lamarck. He writes about De Lamarck
For he quite seriously maintains and tries to prove at length, that the shape of each animal species, the weapons peculiar to it, and its organs of every sort destined for outward use, were by no means present at the origin of that species, but have on the contrary come into being gradually in the course of time and through continued generation, in consequence of the exertions of the animal s will, evoked by the nature of its position and surroundings, through its own repeated efforts and the habits to which these gave rise.
He [Schopenhauer] writes that this could not have been the case. He says there is a simple objection to this. He writes
[…]he overlooks the obvious objection which may be made, that long before the organs necessary for its preservation could have been produced by means of such endeavours as these through countless generations, the whole species must have died out from the want of them. To such a degree may we be blinded by a hypothesis which has once laid hold of us!
He says though
Nevertheless in this instance the hypothesis arose out of a very correct and profound view of Nature : it is an error of genius, which in spite of all the absurdity it contains, still does honour to its originator.
He has praise for De Lamarck while at the same time blames the French for the error. He writes
The true part of it belongs to De Lamarck, as an investigator of Nature ; he saw rightly that the primary element which has determined the animal s organisation, is the will of that animal itself. The false part must be laid to the account of the backward state of Metaphysics in France, where the views of Locke and of his feeble follower, Condillac, in fact still hold their ground and therefore bodies are held to be things in themselves, Time and Space qualities of things in themselves ; and where the great doctrine of the Ideal nature of Space and of Time and of all that is represented in them, which has been so extremely fertile in its results, has not yet penetrated.
He writes that error in De Lamarck’s formulation lies in assuming
the animal to have first been without any clearly defined organs, but also without any clearly defined tendencies, and to have been equipped only with perception.
He continues to write that this idea if carried to its logical end, De Lamarck
ought to have assumed a primary animal which, to be consistent, must have originally had neither shape nor organs, and then proceeded to transform itself according to climate and local conditions into myriads of animal shapes of all sorts, from the gnat to the elephant.
This primary animal, he writes is simply the will to live and as such is not physical but metaphysical. He argues that
the shape and organisation of each animal species has been determined by its own will according to the circumstances in which it wished to live ; not however as a thing physical in Time, but on the contrary as a thing metaphysical outside Time.
The will to live, he writes is
the prius, the thing in itself : its phenomenon (mere representation in the cognitive intellect and its forms of Space and Time) is the animal, fully equipped with all its organs which represent the will to live in those particular circumstances.
and that it is the intellect[knowledge] that is adapted to the mode of life of each animal.
I think that gives a brief outline of his thoughts on the development of organisms to what we have now. And so to end this post, he writes
If, on this occasion, anyone were to raise the question as to whether Nature ought not to have provided insects with at least sufficient intelligence to prevent them from flying into the flame of a candle, our answer would be : most certainly; only she did not know that men would make candles and light them, and natura nihil agit frustra[ Nature does nothing in vain].
* He could have been familiar with the Darwinian theory since this edition is written in 1879, 2 decades after Darwin published the Origins.