Why can't the linker find these functions? [duplicate]

This question already has an answer here:

• Why can templates only be implemented in the header file? 9 answers

In base.h I have:

class MyType {};

class Base {
public:
    Base(MyType const& t);
    MyType t;
};

template <typename T>
class Derived1 : public Base {
public:
    Derived1(MyType const& t);
};

In base.cpp I have

#include "Base.h"

Base::Base(MyType const& t) : t(t) {

}

template<typename T>
Derived1<T>::Derived1(MyType const& t) : Base(t) {

}

In derived.h I have:

#include "Base.h"

class Derived2 : public Derived1<int> {
public:
    Derived2(MyType const& t);
};

In derived.cpp I have:

#include "Derived.h"

Derived2::Derived2(MyType const& t) : Derived1<int>(t){

}

Yet the linker is telling me that

Derived1<int>::Derived1(MyType const&)

is an undefined symbol. Why doesn't the constructor I have for Derived1 count?