“Anti-cruelty societies should be encouraged. The progress of science has suffered little by their existence, and humanity has gained much.”

The Key to Theosophy (1889)

Source: Adams-Jefferson Letters

It appears in multiple anti-vaccination books, all by Trung Nguyen and a co-writer, circa 2018. In September 2021 it is echoed everywhere, including medical-journal articles (on various subjects), with no source given. Right above the Aldous Huxley "quote", these books quote a much earlier anti-vax author. Coincidentally, that author says (elsewhere in his book) this:
Let us admire all-powerful Nature, which is not so easily brought into this serious and lasting disorder by our perverse intrusions; for otherwise, there would hardly have remained a human being alive.
Misattributed
Source: Christian Charles Schieferdecker, in Dr. C. G. G. Nittinger's EVILS OF VACCINATION https://www.google.com/books/edition/Dr_C_G_G_Nittinger_s_Evils_of_Vaccinatio/6CUaAAAAYAAJ, 1856, p.40

Jean-Christophe (1904 - 1912), Journey's End: The Burning Bush (1911)

Source: Dean of the Plasma Dissidents (1988), p. 192.

Partially quoted in René Dubos, Louis Pasteur: Free Lance of Science, Da Capo Press, Inc., 1950. p 396.
Original in French: «La génération spontanée, je la cherche sans la découvrir depuis vingt ans. Non, je ne la juge pas impossible. Mais quoi donc vous autorise à vouloir qu'elle ait été l'origine de la vie? Vous placez la matière avant la vie et vous faites la matière existante de toute éternité. Qui vous dit que, le progrès incessant de la science n'obligera pas les savants, qui vivront dans un siècle, dans mille ans, dans dix mille ans... à affirmer que la vie a été de toute éternité et non la matière.? Vous passez de la matière à la vie parce que votre intelligence actuelle, si bornée par rapport à ce que sera l'intelligence des naturalistes futurs, vous dit qu'elle ne peut comprendre autrement les choses. Qui m'assure que dans dix mille ans on ne considérera pas que c'est de la vie qu'on croira impossible de ne pas passer à la matière? Si vous voulez être au nombre des esprits scientifiques, s, qui seuls comptent, il faut vous débarrasser des idées et des raisonnements a priori et vous en tenir aux déductions nécessaires des faits établis et ne pas accorder plus de confiance qu'il ne faut aux déductions de pures hypothèses." (Pasteur et la philosophie,Patrice Pinet, Editions L'Harmattan, p. 63.
Context: I have been looking for spontaneous generation for twenty years without discovering it. No, I do not judge it impossible. But what allows you to make it the origin of life? You place matter before life and you decide that matter has existed for all eternity. How do you know that the incessant progress of science will not compel scientists to consider that life has existed during eternity, and not matter? You pass from matter to life because your intelligence of today cannot conceive things otherwise. How do you know that in ten thousand years, one will not consider it more likely that matter has emerged from life? You move from matter to life because your current intelligence, so limited compared to what will be the future intelligence of the naturalist, tells you that things cannot be understood otherwise. If you want to be among the scientific minds, what only counts is that you will have to get rid of a priori reasoning and ideas, and you will have to do necessary deductions not giving more confidence than we should to deductions from wild speculation.

Letter on behalf of PETA Asia to Jacob Zuma, as quoted in "‘Tradition Is Not an Excuse for Cruelty’" https://www.peta.org/blog/tradition-excuse-cruelty/, PETA (6 November 2009)
2001-2010

Source: Letter to his daughter (1978), p. 15.
Context: What gift can I give you from this cell out of which my hand cannot pass? I give you the hand of the people. What celebration can I hold for you? I give you the celebration of a celebrated memory and a celebrated name. You are the heir to and inheritor of the most ancient civilization. Please make your full contribution to making this ancient civilization the most progressive and the most powerful. By progressive and powerful I do not mean the most dreaded. A dreaded society is not a civilized society. The most progressive and powerful society in the civilized sense, is a society which has recognized its ethos, and come to terms with the past and the present, with religion and science, with modernism and mysticism, with materialism and spirituality; a society free of tension, a society rich in culture. Such a society cannot come with hocus-pocus formulas and with fraud. It has to flow from the depth of a divine search. In other words, a classless society has to emerge but not necessarily a Marxist society. The Marxist society has created its own class structure.

"Jobs: the basic truths we have cast aside", The Times (7 August 1984), p. 10
1980s and later