Sphere doesn't show| Mathematica

Question

Hi I am creating simulation of Brown movement on a 3d surface, so I tried to link 3dplot and animate function. Everything is compiling without any errors but my problem is that the Sphere which I want to move randomly on the surface do not show (nor animate of course). What could be the problem? Here is the code:

Z[x_, y_] :=  4 x + 3 y - 2         
r = 0.05
Bok = 100
X[1] = RandomReal[{0, Bok}]
Y[1] = RandomReal[{0, Bok}]
Z[1] := Z[X1, Y1]
P[1] = Point[{X1, Y1, Z1}]

For[i = 1, i < 1000, i++, X[i + 1] = X[i] + RandomChoice[{0.1, -0.1}];
Y[i + 1] = Y[i] + RandomChoice[{0.1, -0.1}];
Z[i + 1] = Z[X[i + 1], Y[i + 1]];
P[i + 1] := Table[ Point[{X[i + 1], Y[i + 1], Z[i + 1]}], {i, 1000}];
(*Print[P[i+1]]*)
]

Animate[
Show[
Plot3D[Z[x, y], {x, 0, Bok}, {y, 0, Bok}, 
ColorFunction -> "DarkRainbow", Mesh -> None],
Graphics3D[
Black, Sphere[P[i], r]
]
],
{i, 1, 1000}
]

Answer 1

A start for you...

funcZ[x_, y_] := 4 x + 3 y - 2
r = 0.05;
Bok = 100;
X[1] = RandomReal[{0, Bok}];
Y[1] = RandomReal[{0, Bok}];
Z[a_] := funcZ[X[a], Y[a]]
P[1] = {X[1], Y[1], Z[1]};

For[i = 1, i < 1000, i++,
 X[i + 1] = X[i] + RandomChoice[{0.1, -0.1}];
 Y[i + 1] = Y[i] + RandomChoice[{0.1, -0.1}];
 P[i + 1] = {X[i + 1], Y[i + 1], Z[i + 1]}]

ar = Array[P, 1000];

plotrange = Through[{Min, Max}[#]] & /@ Transpose[ar];

g = Graphics3D[{Black, Point[Array[P, 1000]]}, BoxRatios -> {1, 1, 1}]