The most important thing to know about Secondary Causes, is that they cannot cause anything that was not first caused and created, ex nihilo, and in toto by the Primary Efficient and Final cause of all things during the first Six Days of the World, when God established His Order of Creation to be preserved throughout time in the Order of Generation.
It is in this Order of Generation that Secondary Causes perpetuate by physical transmission or reproduction - the Substantial Forms that God created -- as only He can, so create the original and immutable forms of all corporeal beings. Secondary causes have received the power of agency in order to transmit the Substantial Form - which Form determines all the processes or functions of that form.
Dr. Stephen M. Barr, in his 2011 pamphlet, SCIENCE AND RELIGION: THE MYTH OF CONFLICT, - page 39 quotes eminent Catholic scientist Peter E. Hodgson on the view of John Henry Newman:
... Newman believed that the Creator lets His work develop through secondary causes - which have imparted certain laws to matter... millions of ages ago, which have surely and precisely worked out.... in the course of those long ages, those effects which He, from the first, proposed... Thus, Mr. Darwin's theory need not be atheistical, be it true or not.... it may simply be suggesting a large idea of Divine Prescience and Skill. (Ignatius Press, 2011.)
The phrase "be it true or not", saves Newman from total commitment to Darwinism, but not Dr. Hodgson nor Dr. Barr, both of whom are, to all appearances from their published works, committed theistic evolutionists. But it all hinges upon a colossal error in the life sciences, leading to a very false idea, even heretical, about the nature of God. The error in the natural sciences is that functions and processes operate in search of form, as it were. But the opposite is true. It is absolutely necessary, biologically, physically, metaphysically and theologically that Form come before process and function. It is a case of Dr. Michael Behe's mouse trap. All the parts are necessary if it is to catch a mouse. Now Dr.Barr says several very good and true things in this little pamphlet - as when he defines miracles as "extraordinary events.... (that presupposes that there is a natural order that determines what is naturally possible and what is not..." (pp.10-11)
Now it is just as certain that secondary causes presuppose a first, Primary Efficient and Final Cause, as it is -- that miracles presuppose a natural order.... that a miracle temporarily suspends. Because the natural order, and its in-built laws, are but the Order of Creation described as established in the First Six Days of the World and preserved, with accidental variations, throughout time in the Order of Generation. But Dr. Barr commits a colossal error - and he certainly cannot document it when he says:
For the Catholic Church, the physical processes by which plants and animals formed, like those by which stars and planets formed, are a matter of natural "secondary causes" and for the natural sciences, not theology, to concern themselves with. (page 40.) I underlined these words because this is the crucial error: science can only see the processes that take place by and in the Forms of created beings, that is, processes that only began once the Forms had been created in toto by God, during the First Six Days of the World and then transmitted by reproduction in the Order of Generation. These original forms, the created kinds of Genesis One are immutable creations, as history proves, except for minor variations. Some variations, such as those within the canine and feline kinds, may seem more than minor...- from lion to domestic cat or from Jack London's "White Fang" - to my pet Fido - but they can interbreed, and it is for the taxonomist to discover these boundaries of the Genesis kind. But Saint Thomas is very emphatic - that while it may not be for theology to oversee the ordinary work of the scientists, it is for theology to judge of the scientists conclusions - in the light of Divine Revelation. (See question 1 of the Summa - where the sciences are all discussed in relation to Theology.)
And I must add that it is most certainly - not - "the Catholic Church" - that teaches anything remotely resembling Darwinian evolution by secondary causes. St.Thomas is equally clear and emphatic that in creation - God used no instrumental or secondary causes - because creation of all the first substantial forms was a work done by Acts of God alone. No processes at all were involved in God's Creative Acts. He spoke and they were. He commanded and they came into existence.(Psalm 32.) This evolutionism propagated by prominent men of science - like Dr.Steven Barr and Jesuit Father Robert Spitzer, is the major error of the synthesis of all heresies that Pope St. Pius X condemned as Modernism in 1907 - in his encyclical Pascendi and his syllabus Lamentabili. What scientists - like Dr.Barr observe through their high-powered telescopes is not the formation of anything -- but rather the motions of disintegration - things coming apart - not coming together to form intelligible objects. God did that in the beginning, just as Genesis One narrates. God created all things ex nihilo and in toto. The in toto means that all things issued from His Creative Word - instantaneously, fully formed, mature and ready to begin to function according to their created Forms - Substances - Natures and Kinds.
Genesis 1-3 is full of all the real, scientific knowledge - any professional scientist could possibly wish for. But he must have eyes to see and ears to hear, as Our Divine Lord so often admonishes.
(For the places in the Summa of St. Thomas, please see this writer's, THIRTY THESES AGAINST THEISTIC EVOLUTION.)