|First page to Ludwig Boltzmann's famous 1886 lecture on the second law of thermodynamics.|
Struggle for entropy
The most-quoted passage from the lecture, stated rather cryptically, is that life is a struggle for entropy: 
“The general struggle for existence of animate beings is not a struggle for raw materials – these, for organisms, are air, water and soil, all abundantly available – nor for energy, which exists in plenty in any body in the form of heat Q, but of a struggle for entropy, which becomes available through the transition of energy from the hot sun to the cold earth.”
To note, there is a bit of translation issue in this quote as to whether Boltzmann spoke about entropy or free energy as the focus of the "struggle", as discussed further below.
Another famous statement made by Boltzmann was that he gave his opinion that in the future, the 19th century will be seen as the century of the theory of evolution and of the mechanical theory of heat: 
“If you ask me about my innermost conviction whether our century will be called the century of iron or the century of steam or electricity, I answer without hesitation: it will be called the century of the mechanical view of nature, the century of Darwin.”
In basing his ideas on struggle Boltzmann gives his opening opinion in his essay that the achievement that will put its stamp on the 19th century will be English naturalist Charles Darwin’s 1859 Origin of Species.
Curiously, in the works of both German chemist Walther Nernst (1893) and Romanian mathematician Nicholas Georgescu (1971), they both commented that in German version of this essay Boltzmann argued to the effect that "free energy", rather than entropy, which is the object of the struggle in life. Nernst comments: 
“Strife is not for the component substances, for these component substances of all organism, as air, water, and earth, are abundant; neither is the strife for energy as such, for this occurs in abundance, as the heat content of matter of our environment; but strife is for the free energy available for the performance of work.”
Likewise, Georgescu-Roegen comments: 
“Life does not feed on mere matter and mere energy but—as Schrodinger aptly explained—on low entropy. The seed of this idea goes back to Boltzmann who was the first to point out that free energy is the object of the struggle for life.”
It is difficult, however, to actually find Boltzmann using the term "free energy" in this essay, meaning that this may be a German→English translation issue? He does, to note, cite Hermann Helmholtz (who introduced the term free energy as the measure of affinity in 1882) and discuss energy set free in the context of chemical affinity.
1. (a) Boltzmann, Ludwig. (1886). "The Second Law of Thermodynamics" (pgs. 14-32; struggle quote, pg. 24, affinity and energy set free, pgs. 26-27). In B. McGinness, ed., Ludwig Boltzmann: Theoretical physics and Philosophical Problems: Selected Writings. Dordrecht, Netherlands: D. Reidel, 1974.
(b) Boltzmann, Ludwig. (1886). “Der Zweite Hauptsatz der Mechanischen Wärmetheorie” (The Second Law of Thermodynamics), pg. 21. Vortrag., Wien bei Gerold; Address to a formal meeting of the Imperial Academy of Science, Popular Schriften, Essay 3, May 29.
3. (a) Nernst, Walther. (1895). Theoretical Chemistry: from the Standpoint of Avogadro’s Rule & Thermodynamics (Book III, ch. VII: Photo-Chemistry, pgs. 617-; section: Theory of Photo-Chemical Action, pgs. 626-28). MacMillan and Co.
(b) Boltzmann, Ludwig. (1886). “Der Zweite Hauptsaz der Mech. Warmeth. (The Second Law of Thermodynamics), pg. 21. Vortrag., Wien bei Gerold.
4. (a) Georgescu-Roegen, Nicholas. (1966). Analytical Economics: Issues and Problems (pg. 82). Harvard University Press.
(b) Georgescu-Roegen, Nicholas. (1971). The Entropy Law and the Economic Process (pg. 192). Cambridge, Massachusetts: Harvard University Press.
5. Gallavotti, Giovanni, Reiter, Wolfgang L., and Yngvason, Jakob. (2008). Boltzmann’s Legacy (pg. 217). European Mathematical Society.