So far as my limited understanding of Marxism extends, according to Marx capitalism contains the seeds of its own destruction. What I think he was essentially getting at is that, as Wonderpup has already explained, the endless competition between industrialist bosses would inevitably lead to a collapse of their customer base. This, in turn, would lead to civil unrest and, eventually, revolution.
The reason for the above is the need for profit. In its most essential form, profit may be defined as a situation whereby you exit an economic transaction with more resources than when you went in. In order for anyone to extract a profit, a loss must be incurred by someone, or some thing, somewhere else. Either the supplier of the raw materials makes a loss, or the manufacturer makes a loss, or the retailer makes a loss, or the customer makes a loss, or the workers all the way up the production chain makes a loss. Someone or something has to make a loss. The above leads to the paradox of a situation where the ideal for any capitalist would be to do away with all workers and automate all production since this would prove to be the most profitable arrangement. However, those workers are also their customers. Thus, in doing away with them, they have destroyed their customer base's capacity to spend money on buying their products.
Growth is the only thing that has allowed capitalism to paper over this fundamental flaw in its heart and so allow it to last for as long as it has. Which, in historical terms, is not very long at all. This is not to say that any system of human organisation would not have run up against the same resource buffers. It's just that capitalism has proved to be very efficient at getting us here. In this sense, whilst resources are available to enable growth to occur, the "loss" is incurred by the Earth itself. Now that growth is no longer physically possible due to resource constraints in the context of a world where every man and his dog (understandably) now wants a Western lifestyle, the days of win/win capitalism are over. In a static or contracting economy all profits must be balanced with losses.
Or, to put all of the above more succinctly: Capitalism does not work in a static or contracting economy unless one is prepared to accept considerable levels of bankruptcies/social unrest etc. Fractional reserve banking (FRB) stimulates and amplifies economic growth by borrowing from the future and so avoids, for a time, the inherent flaw in capitalism. The earth has finite amounts of easily accessible industrial resources eventually resulting in a situation where growth has stopped. Capitalism's inherent flaw is exposed.
I should expand, here, on a subsidiary but very important factor. That of fractional reserve banking. FRB, as I am sure many of you on here understand, is a financial process whereby if I go to a bank and borrow money from them I must pay it back with the monetary fruits of my own future productive activities. The money that the domestic bank lent to me, they also borrowed from a commercial bank. The commercial bank lent that money into existence. Or, rather, multiplied it into existence on the back of an initial injection into the system of a small amount of base money by a CB. Thus, ultimately, central banks/commercial lenders decide how much is lent into existence to you and me based on an assumption of how much productive activity will be happening in the future such that the lending can be repaid. Of course, the reality of how the credit trickles down through the system is far more tortuous and indirect than the schematic, above. But, in principle, this is how our FRB "money" supply grows.
FRB, in the above conceptualisation, may be seen as borrowing from the future. Or, if you prefer, FRB is a form of the lending of the future to today. The semantics of the description matter less than the actuality of the process. FRB has been yet another weapon in the armoury of capitalism whereby the losses incurred today can be papered over. In FRB's case, by drawing down profits from the future. Thus, economic growth is accelerated under an FRB system of money creation.
However, there is a fundamental truth we all know in our guts and which science has merely formalised. Namely, that there is no such thing as free lunch. As long as there are a large amount of resources, we can pretend, for a while, that there is. Once we hit the limits of those resources, though, FRB stops working as a consequence of the future failing to honour the FRB promises of today. At that point, we get economic collapse (the current crisis is the beginning of that collapse). From now on in, in order for profits to be maintained, someone has to lose for someone else to gain.
Perpetual economic growth is over. A money supply that is increased over time via FRB credit is over. Capitalism is over.
The future will be either some form of socialism or a return to serfdom for the majority